Piano music for a duo (4)
I believe the answer is:
pair
'a duo' is the definition.
('pair' can be a synonym of 'duo')
'piano music' is the wordplay.
'piano' becomes 'p'.
'music' becomes 'air' (air is a kind of music**).
'p'+'air'='PAIR'
'for' acts as a link.
(Other definitions for pair that I've seen before include "Twosome" , "players of duet?" , "Brace, couple" , "Two of a similar type" , "Set of two" .)