Complain when a jam jar is put on the piano (4)
I believe the answer is:
carp
'complain' is the definition.
(carp can informally mean to complain or whinge)
'a jam jar is put on the piano' is the wordplay.
'a jam jar' becomes 'car' (I can't explain this - if you can you should believe this answer much more).
'is put on' means one lot of letters go next to another.
'the piano' becomes 'p' (musical abbreviation).
'car'+'p'='CARP'
'when' is the link.
Can you help me to learn more?
(Other definitions for carp that I've seen before include "Complain continually" , "Fish often kept in ponds" , "Fish - make captious criticism" , "Cavil - fish" , "Keep complaining" .)