Tune on piano for twosome (4)
I believe the answer is:
pair
'twosome' is the definition.
('pair' can be a synonym of 'twosome')
'tune on piano' is the wordplay.
'tune' becomes 'air' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'piano' becomes 'p'.
'air' put after 'p' is 'PAIR'.
'for' acts as a link.
(Other definitions for pair that I've seen before include "Cereal used as food" , "Twins" , "Agreement to avoid division" , "how braces come" , "Couple - wed" .)