Couple's piano tune (4)
I believe the answer is:
pair
'couple's' is the definition.
('pair' can be a synonym of 'couple')
'piano tune' is the wordplay.
'piano' becomes 'p'.
'tune' becomes 'air' (I've seen this before).
'p'+'air'='PAIR'
(Other definitions for pair that I've seen before include "Husband and wife" , "Two of a similar type" , "item" , "Cereal used as food" , "Duo" .)