Knock worthless piano (4)
I believe the answer is:
bump
'knock' is the definition.
(I know that knock can be written as bump)
'worthless piano' is the wordplay.
'worthless' becomes 'bum' (bum can mean of poor quality).
'piano' becomes 'p' (musical abbreviation).
'bum'+'p'='BUMP'
(Other definitions for bump that I've seen before include "Light blow or jolting collision" , "Thud down, into" , "Lump caused by a blow" , "Hit" , "Slang for kill - . . . . off" .)