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.
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   2. To utter rapidly and inconsiderately. --Feltham.
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   To clack wool, to cut off the sheep's mark, in order to
      make the wool weigh less and thus yield less duty. [Eng.]
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