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]