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]

WordNet (r) 3.0 (2006):

clack valve
    n 1: a simple valve with a hinge on one side; allows fluid to
         flow in only one direction [syn: clack valve, clack,
         clapper valve]