Rapper's critic? (7)

I believe the answer is:

knocker

I believe this is a double definition.

'rapper's' is the first definition.
(eg someone knocking or rapping on a door)

'critic?' is the second definition.
(someone who knocks or criticises things)

(Other definitions for knocker that I've seen before include "Door banger -- detractor" , "It bangs on a door" , "Hinged device on a door" , "means to announce house call" , "Critic" .)

I've seen this clue in The Telegraph.

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