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.
      [1913 Webster]

   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]