Piano music needed for the duet (4)
I believe the answer is:
pair
'the duet' is the definition.
('pair' can be a synonym of 'duet')
'piano music needed' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'music' becomes 'air' (air is a kind of music).
'needed' says to put letters next to each other.
'p'+'air'='PAIR'
'for' acts as a link.
(Other definitions for pair that I've seen before include "2" , "item" , "Couple - wed" , "Twins" , "flight of stairs" .)