Duet for piano melody (4)
I believe the answer is:
pair
'duet' is the definition.
(I know that duet can be written as pair)
'piano melody' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'melody' becomes 'air' (air can mean a tune or melody).
'p'+'air'='PAIR'
'for' acts as a link.
(Other definitions for pair that I've seen before include "E.g. two aces; mate" , "Put together - two of a kind" , "Duo" , "item" , "Husband and wife" .)