Group of six (5)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">group of six</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>
(Another definition for hexad that I've seen is " Group or series of six".)
(Other definitions for sextet that I've seen before include "band" , "Group of musicians" , "Piece for six" , "A group of six singers" , "Group of six (X, not S)" .)