Proper time for a piece of music (4)
I believe the answer is:
duet
'a piece of music' is the definition.
(duet is a kind of piece of music)
'proper time' is the wordplay.
'proper' becomes 'due' (I've seen this before).
'time' becomes 't'.
'due'+'t'='DUET'
'for' is the link.
(Other definitions for duet that I've seen before include "Two singers performing together" , "Work for two players" , "Composition for two performers" , "Two-piece" , "Music for two" .)