The Collaborative International Dictionary of English v.0.48:

Clack \Clack\, v. t.
   1. To cause to make a sudden, sharp noise, or succession of
      noises; to click.
      [1913 Webster]

   2. To utter rapidly and inconsiderately. --Feltham.
      [1913 Webster]

   To clack wool, to cut off the sheep's mark, in order to
      make the wool weigh less and thus yield less duty. [Eng.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Clack \Clack\, n. [Cf. F. claque a slap or smack, MHG. klac
   crack, W. clec crack, gossip. See Clack, v. t.]
   1. A sharp, abrupt noise, or succession of noises, made by
      striking an object.
      [1913 Webster]

   2. Anything that causes a clacking noise, as the clapper of a
      mill, or a clack valve.
      [1913 Webster]

   3. Continual or importunate talk; prattle; prating.
      [1913 Webster]

            Whose chief intent is to vaunt his spiritual clack.
                                                  --South.
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   Clack box (Mach.), the box or chamber in which a clack
      valve works.

   Clack dish, a dish with a movable lid, formerly carried by
      beggars, who clacked the lid to attract notice. --Shak.
      [1913 Webster]

   Clack door (Mining), removable cover of the opening through
      which access is had to a pump valve.

   Clack valve (Mach.), a valve; esp. one hinged at one edge,
      which, when raised from its seat, falls with a clacking
      sound.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Clack \Clack\, v. i. [imp. & p. p. Clacked; p. pr. & vb. n.
   Clacking.] [Prob. of imitative origin; cf. F.claquer to
   clap, crack, D. klakken, MHG. klac crack, Ir. clagaim I make
   a noise, ring. Cf. Clack, n., Clatter, Click.]
   1. To make a sudden, sharp noise, or a succesion of such
      noises, as by striking an object, or by collision of
      parts; to rattle; to click.
      [1913 Webster]

            We heard Mr.Hodson's whip clacking on the ahoulders
            of the poor little wretches.          --Thackeray.
      [1913 Webster]

   2. To utter words rapidly and continually, or with
      abruptness; to let the tongue run.
      [1913 Webster]

WordNet (r) 3.0 (2006):

clack
    n 1: a sharp abrupt noise as if two objects hit together; may be
         repeated [syn: clack, clap]
    2: a simple valve with a hinge on one side; allows fluid to flow
       in only one direction [syn: clack valve, clack, clapper
       valve]
    v 1: make a rattling sound; "clattering dishes" [syn: clatter,
         clack, brattle]
    2: make a clucking sounds, characteristic of hens [syn: cluck,
       click, clack]
    3: speak (about unimportant matters) rapidly and incessantly
       [syn: chatter, piffle, palaver, prate, tittle-
       tattle, twaddle, clack, maunder, prattle, blab,
       gibber, tattle, blabber, gabble]

Moby Thesaurus II by Grady Ward, 1.0:

151 Moby Thesaurus words for "clack":
   Klaxon, babble, babblement, bavardage, bibble-babble, bicker, blab,
   blabber, blah-blah, blather, blether, blethers, boiler factory,
   boiler room, brattle, bull-roarer, cackle, call, caquet,
   caqueterie, carol, catcall, caw, chat, chatter, cheep, cherry bomb,
   chirk, chirp, chirr, chirrup, chitter, chitter-chatter, chuck,
   clacker, clacket, clapper, clatter, clatter about, click, clitter,
   clitterclatter, cluck, clunter, cock-a-doodle-doo, coo, crack,
   cracker, cracking, crackle, crackling, crepitate, crepitation,
   cricket, croak, cronk, crow, cuckoo, decrepitate, decrepitation,
   dither, drum, firecracker, gab, gabble, gaggle, gas, gibber,
   gibble-gabble, go on, gobble, gossip, guff, guggle, gush, haver,
   honk, hoo, hoot, horn, hot air, idle talk, jabber, jaw, mere talk,
   natter, noisemaker, nonsense talk, palaver, patter, peep, pip,
   pipe, pour forth, prate, prating, prattle, prittle-prattle, quack,
   quidnunc, racket, ramble on, rattle, rattle around, rattle on,
   rattlebox, rattletybang, rattling, reel off, roll, ruckle,
   rumormonger, run on, scandalmonger, scold, shatter, sieve, sing,
   siren, sizzling, snap, snapper, snapping, spit, spitting, spout,
   spout off, squawk, steam whistle, tabby, talebearer, talk away,
   talk nonsense, talk on, talkee-talkee, telltale, ticktack,
   tittle-tattle, trill, twaddle, twattle, tweet, twit, twitter,
   waffle, warble, whistle, whizgig, whizzer, yak, yakkety-yak