Sharp blow (3)
I believe the answer is:
rap
'sharp blow' is the definition.
(rap can mean a sharp hit or blow)
This is all the clue.
(Other definitions for rap that I've seen before include "Knock for par" , "Knock; type of rhythmical speech to music" , "Sharp knock; rebuke" , "Knock, tap" , "Administer sharp blow, being below par" .)