The Collaborative International Dictionary of English v.0.48:

Knacker \Knack"er\, n.
   1. One who makes knickknacks, toys, etc. --Mortimer.
      [1913 Webster]

   2. One of two or more pieces of bone or wood held loosely
      between the fingers, and struck together by moving the
      hand; -- called also clapper. --Halliwell.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Clapper \Clap"per\, n.
   1. A person who claps.
      [1913 Webster]

   2. That which strikes or claps, as the tongue of a bell, or
      the piece of wood that strikes a mill hopper, etc. See
      Illust. of Bell.
      [1913 Webster]

   Clapper rail (Zool.), an Americam species of rail (Rallus
      scepitans).
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Clapper \Clap"per\, n. [F. clapier.]
   A rabbit burrow. [Obs.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

clapper
    n 1: someone who applauds [syn: clapper, applauder]
    2: a mobile mass of muscular tissue covered with mucous membrane
       and located in the oral cavity [syn: tongue, lingua,
       glossa, clapper]
    3: metal striker that hangs inside a bell and makes a sound by
       hitting the side [syn: clapper, tongue]