Knock back, as usual (3)

I believe the answer is:

rap

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'knock' is the definition.
(I know that knock can be written as rap)

I cannot understand how the rest of the clue works.

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(Other definitions for rap that I've seen before include "Short quick tap" , "Knock - punishment - monologue to music" , "Rhyming monologue recited to rock music below par" , "taken by those held to blame?" , "Criminal charge" .)

I've seen this clue in the Evening Standard.

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