Bounce backside on piano (4)
I believe the answer is:
bump
'bounce' is the definition.
(I've seen this before)
'backside on piano' is the wordplay.
'backside' becomes 'bum' ('bum' can be a synonym of 'backside').
'on' says to put letters next to each other.
'piano' becomes 'p'.
'bum'+'p'='BUMP'
(Other definitions for bump that I've seen before include "Raised mark left by blow" , "Collide - protuberance" , "Knock - collide" , "Lump caused by a blow" , "Evidence of pregnancy - knock (into)" .)