Sharp blow (3)
'sharp blow' is the definition.
(rap can mean a sharp hit or blow)
This is the entire 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" .)
(Other definitions for rabbit punch that I've seen before include "It may knock you out" , "hit" , "Blow to back of neck" , "What causes a pain in the neck" , "Sharp chop to the back of the neck" .)