Half of a sextet (4)
I believe the answer is:
trio
'half of a sextet' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for trio that I've seen before include "father, spirit and son" , "Three of a kind" , "Musical group" , "(Composition for) three performers" , "Bee Gees, Supremes or Andrews Sisters, say" .)