Group of six (6)
'group of six' is the definition.
(I've seen this in another clue)
This is all the clue.
(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)" .)