The Collaborative International Dictionary of English v.0.48:

Clash \Clash\, v. t.
   To strike noisily against or together.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Clash \Clash\ n.
   1. A loud noise resulting from collision; a noisy collision
      of bodies; a collision.
      [1913 Webster]

            The roll of cannon and clash of arms. --Tennyson.
      [1913 Webster]

   2. Opposition; contradiction; as between differing or
      contending interests, views, purposes, etc.
      [1913 Webster]

            Clashes between popes and kings.      --Denham.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Clash \Clash\, v. i. [imp. & p. p. Clashed; p. pr. & vb. n.
   Clashing.] [Of imitative origin; cf. G. klatschen, Prov. G.
   kleschen, D. kletsen, Dan. klaske, E. clack.]
   1. To make a noise by striking against something; to dash
      noisily together.
      [1913 Webster]

   2. To meet in opposition; to act in a contrary direction; to
      come onto collision; to interfere.
      [1913 Webster]

            However some of his interests might clash with those
            of the chief adjacent colony.         --Palfrey.
      [1913 Webster]

WordNet (r) 3.0 (2006):

clash
    n 1: a loud resonant repeating noise; "he could hear the clang
         of distant bells" [syn: clang, clangor, clangour,
         clangoring, clank, clash, crash]
    2: a state of conflict between persons [syn: clash,
       friction]
    3: a state of conflict between colors; "her dress was a
       disturbing clash of colors"
    4: a minor short-term fight [syn: brush, clash, encounter,
       skirmish]
    v 1: crash together with violent impact; "The cars collided";
         "Two meteors clashed" [syn: collide, clash]
    2: be incompatible; be or come into conflict; "These colors
       clash" [syn: clash, jar, collide]
    3: disagree violently; "We clashed over the new farm policies"

Moby Thesaurus II by Grady Ward, 1.0:

318 Moby Thesaurus words for "clash":
   Babel, Discordia, Eris, action, aerial combat, affray,
   agree to disagree, altercation, antagonism, antagonize, antipathy,
   appulse, argue, argument, armored combat, bang, bang into, battle,
   battle royal, be antipathetic, be at cross-purposes,
   be at variance, be distinct, be distinguished, be inimical,
   be opposed to, beat against, belch, belligerence, blare, blat, box,
   brawl, bray, break, break off, broil, brunt, brush, bulldozing,
   bullfight, bulling, bump, bump into, burr, burst, bust, buzz,
   cackle, cannon, carambole, carom, carom into, caw, chirr, clamor,
   clang, clangor, clank, clap, clash of arms, clash with, clashing,
   close, cockfight, collide, collision, combat, come into collision,
   come to blows, concuss, concussion, conflict, conflict with,
   confront each other, confute, contend, contention, contest,
   contradict, contrapose, contrast with, contravene, controvert,
   counter, counteract, counterattack, counterbalance, countercheck,
   counterpoise, counterpose, countervail, counterwork, crack,
   crack up, crack-up, crash, crash into, craunch, croak, cross,
   cross swords, crump, crunch, cut and thrust, dash into,
   depart from, despitefulness, deviate from, differ,
   differ in opinion, difference, disaccord, disaccord with,
   disaffinity, disagree, disagree with, disagreement, discord,
   discordance, discordancy, disharmony, dispute, dissent,
   divaricate from, diverge, diverge from, dogfight, duel,
   embroilment, encounter, engagement, enmity, exchange blows,
   exchange of blows, fall foul of, fence, feud, fight, fight a duel,
   fire fight, flap, flop, foul, fracas, fray, fret, friction, gall,
   give and take, give satisfaction, go against, go counter to,
   grapple, grapple with, grate, grind, groan, ground combat, growl,
   grumble, hammering, hand-to-hand combat, hand-to-hand fight,
   harshness, hate, hatred, hit, hit a clinker, hit against,
   hold opposite views, hostility, house-to-house combat, hurt,
   hurtle, impact, impinge, impingement, incompatibility,
   incompatibleness, inharmoniousness, inharmony, interfere with,
   jangle, jar, jar with, jolt, jostle, joust,
   juxtapose in opposition, knock, knock against, lock horns,
   malevolence, malice, malignity, mauling, meet, meet head-on,
   meeting, melee, mere noise, militate against, mischief, mismatch,
   mismate, misunderstand one another, mix it up, naval combat,
   negate, noise, noncooperation, not accord with, not get along,
   not square with, object, offset, onslaught, open conflict, oppose,
   oppugn, passage of arms, percuss, percussion, pitched battle,
   play at cross-purposes, pull different ways, quarrel,
   quarrelsomeness, ramming, rap, rasp, rassle, report, repugnance,
   resist, riot, row, rub, rumble, rumpus, run a tilt, run against,
   run counter to, run into, running fight, scramble, scranch, scrap,
   scrape, scratch, scrimmage, scrunch, scuffle, set off, set-to,
   shock, shoving match, sideswipe, skirmish, slam, slam into, slap,
   slat, sledgehammering, smack, smack into, smash, smash into,
   smash up, smash-up, smashing, snarl, snore, spar, spite,
   spitefulness, splat, squabble, stand apart, stand over against,
   stand-up fight, strained relations, street fight, strike,
   strike against, strive, struggle, swap, swim upstream, tap,
   tauromachy, tension, thrust and parry, thrusting, thwack, tilt,
   tourney, try, tug-of-war, tussle, twang, unharmoniousness,
   unpleasantness, unstring, untune, vary, wage war, wallop, war,
   whack, wham, whap, whomp, whop, work against, wrangle, wrestle