Part of joint a music group set up (5)
I believe the answer is:
tenon
'part of joint' is the definition.
(type of wooden joint)
'a music group set up' is the wordplay.
'a music group' becomes 'nonet' (group of nine musicians).
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
'nonet' written backwards gives 'TENON'.
(Other definitions for tenon that I've seen before include "Shaped end of piece of wood for slotting into a hole." , "Mortise extension" , "Piece to fit into mortise" , "Mortise insert" , "Tongue of wood, fits into mortise" .)