Knock a style of music (3)
I believe the answer is:
rap
I believe this is a double definition.
'knock' is the first definition.
(synonyms)
'a style of music' is the second definition.
(I've seen this before)
(Other definitions for rap that I've seen before include "Rhyming monologue recited to rock music below par" , "Type of popular music" , "Sharp blow; criticism" , "Administer sharp blow, being below par" , "Censure" .)