Duo's quiet melody (4)
I believe the answer is:
pair
'duo's' is the definition.
(synonyms)
'quiet melody' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'melody' becomes 'air' (synonyms).
'p'+'air'='PAIR'
(Other definitions for pair that I've seen before include "Duo" , "Husband and wife" , "players of duet?" , "Two ducks in one game of cricket" , "Agreement to avoid division" .)