Backing group of musicians in joint (5)
I believe the answer is:
tenon
'joint' is the definition.
(I've seen this in other clues)
'backing group of musicians' is the wordplay.
'backing' is a reversal indicator.
'group of musicians' becomes 'nonet' (a nonet requires nine musicians).
'nonet' in reverse letter order is 'TENON'.
'in' is the link.
(Other definitions for tenon that I've seen before include "Tongue of wood for joint" , "piece of carpentry?" , "Wooden projection shaped to fit in a mortise joint" , "joiner" , "Projection used in a mortise joint" .)