Piece for two
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">piece for two</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for duet that I've seen before include "Performance by company, perhaps" , "two share it" , "Music for two" , "Work for two performers" , "Two singers performing together" .)