Set melody on piano (4)
I believe the answer is:
pair
'set' is the definition.
(I know that pair is a type of set)
'melody on piano' is the wordplay.
'melody' becomes 'air' (synonyms).
'on' is a charade indicator (letters next to each other).
'piano' becomes 'p' (musical abbreviation).
'air' put after 'p' is 'PAIR'.
(Other definitions for pair that I've seen before include "Couple - wed" , "E.g. two aces; mate" , "Cereal used as food" , "item" , "Two of a kind, as in cards" .)