Musical piece for two (4)
I believe the answer is:
duet
'musical piece for two' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for duet that I've seen before include "Composition for two performers" , "Work for two players" , "Work for two performers" , "Musical piece for two performers" , "Piece for brace" .)