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]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "clapper":
   Klaxon, bell, boiler factory, boiler room, bull-roarer, catcall,
   cherry bomb, chimes, church bell, clack, clacker, cowbell, cracker,
   cricket, dinner bell, dinner gong, doorbell, fire bell,
   firecracker, gong, gong bell, hand bell, horn, jingle bell,
   noisemaker, passing bell, rattle, rattlebox, sacring bell,
   sheepbell, siren, sleigh bell, snapper, steam whistle,
   telephone bell, ticktack, tintinnabulum, tongue, triangle, whistle,
   whizgig, whizzer