Shift worthless piano (4)
I believe the answer is:
bump
'shift worthless piano' is the definition.
The answer and definition can be both related to natural events as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'worthless piano' is the wordplay.
'worthless' becomes 'bum' (bum can mean of poor quality).
'piano' becomes 'p' (musical abbreviation).
'bum'+'p'='BUMP'
Can you help me to learn more?
(Other definitions for bump that I've seen before include "striking" , "Bounce" , "Knock - collide" , "Lump caused by a blow" , "Collide - protuberance" .)