Duet involving piano melody (4)
I believe the answer is:
pair
'duet' is the definition.
('pair' can be a synonym of 'duet')
'piano melody' is the wordplay.
'piano' becomes 'p'.
'melody' becomes 'air' ('air' can be a synonym of 'melody').
'p'+'air'='PAIR'
'involving' is the link.
(Other definitions for pair that I've seen before include "Duo" , "Agreement to avoid division" , "Put together - two of a kind" , "Cereal used as food" , "2" .)