Half a sextet (4)
I believe the answer is:
trio
'half 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 "Chamber group" , "Riot (anag)" , "Beethoven's 'Archduke', for instance" , "Work for three" , "Ensemble musical piece, eg Beethoven's 'Archduke'" .)