Door rapper (7)
I believe the answer is:
knocker
'door rapper' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for knocker that I've seen before include "It bangs on a door" , "Something on door; one continually finding fault" , "means to announce house call" , "Alternative to door bell" , "Hinged device on a door" .)