Retired musical group's section of joint (5)
I believe the answer is:
tenon
'section of joint' is the definition.
(part of a wooden joint)
'retired musical group's' is the wordplay.
'retired' shows that the letters should be reversed in order.
'musical group' becomes 'nonet' (I've seen this before).
'nonet' in reverse letter order is 'TENON'.
(Other definitions for tenon that I've seen before include "Projection used in a mortise joint" , "Part of woodworking joint" , "Type of small saw" , "Connection" , "Joint - tonne (anag)" .)