Small group, owing to time (4)
I believe the answer is:
'small group' is the definition.
'owing to time' is the wordplay.
'owing' becomes 'due' (similar in meaning).
'to time' becomes 't' (abbreviation).
(Other definitions for duet that I've seen before include "Composition for two performers" , "Work for a couple" , "Two singers performing together" , "Paired performance" , "Performance by two" .)