Four duos (5)
I believe the answer is:
octet
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">four duos</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 octet that I've seen before include "Lines of verse" , "Eight people together" , "Piece for eight players" , "music group" , "A group of eight instruments or voices" .)