Part of joint a music group knocked back (5)
I believe the answer is:
tenon
'part of joint' is the definition.
(type of wooden joint)
'a music group knocked back' is the wordplay.
'a music group' becomes 'nonet' (group of nine musicians).
'knocked back' is a reversal indicator.
'nonet' written backwards gives 'TENON'.
(Other definitions for tenon that I've seen before include "Part of a wooden joint" , "Part of joint that fits in mortise" , "Piece to fit into mortise" , "Joint - tonne (anag)" , "Saw; joint" .)