Music for two (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">music for two</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for duet that I've seen before include "Music for two" , "Work for a couple" , "Composition for two performers" , "Piece for brace" , "Performance by company, perhaps" .)