Source of criticism for rapper (7)
I believe the answer is:
knocker
This clue is a double definition.
'source of criticism' is the first definition.
(someone criticising or knocking something)
'rapper' is the second definition.
(eg someone knocking or rapping on a door)
'for' is the link.
(Other definitions for knocker that I've seen before include "Something on door; one continually finding fault" , "means to announce house call" , "Fault-finder; door item" , "One strongly criticising" , "Alternative to door bell" .)