Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. To utter rapidly and inconsiderately. --Feltham.

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