Piano melody — or duo (4)
I believe the answer is:
pair
'duo' is the definition.
(I know that duo 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'
'or' is the link.
(Other definitions for pair that I've seen before include "players of duet?" , "flight of stairs" , "Two ducks in one game of cricket" , "Twins" , "how braces come" .)