Two-score (4)
(Other definitions for duet that I've seen before include "Music for two" , "Paired performance" , "Work for a couple" , "Piece for brace" , "A song sung by two" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">two-score</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for forty that I've seen before include "-- winks, a short sleep" , "number of penitential days" , "Kip" , "Two score" , "Two-score" .)