Gentle melody for a duet (4)
I believe the answer is:
pair
'duet' is the definition.
(synonyms)
'gentle melody' is the wordplay.
'gentle' becomes 'p' (I've seen this in other clues).
'melody' becomes 'air' (air can mean a tune or melody).
'p'+'air'='PAIR'
'for a' acts as a link.
(Other definitions for pair that I've seen before include "flight of stairs" , "players of duet?" , "Cereal used as food" , "2" , "Duo" .)