Knock sharply (3)
I believe the answer is:
rap
'knock sharply' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rap that I've seen before include "black music is a > blow" , "Form of music with chanted rhyming lyrics" , "Rhyming monologue recited to rock music below par" , "Strike; music genre" , "Sharp blow; criticism" .)