A clicker was the lead compositor in a traditional print shop who received the copy, distributed it among the typesetters in a companionship, and then assembled their finished galleys into complete pages. The clicker was responsible for locking the type into the chase, adding the running heads and folios, and ensuring the final form was ready for the press.
Heard on the job
Hand your finished takes to the clicker so he can start making up the first sheet.