The piano key on the left, chum (3)
I believe the answer is:
pal
'chum' is the definition.
(synonyms)
'the piano key on the left' is the wordplay.
'the piano key' becomes 'p' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' is a charade indicator (letters next to each other).
'the left' becomes 'al' (I've seen this in other clues).
'p'+'al'='PAL'
Can you help me to learn more?
(Other definitions for pal that I've seen before include "Close friend" , "Mate, informal friend" , "Bud" , "Brother" , "China" .)