Musical group turning up — one in the groove? (5)
I believe the answer is:
tenon
'one in the groove?' is the definition.
(as in a tenon joint)
'musical group turning up' is the wordplay.
'musical group' becomes 'nonet' (group of nine musicians).
'turning up' is a reversal indicator.
'nonet' in reverse letter order is 'TENON'.
(Other definitions for tenon that I've seen before include "Wood fitting into a mortise" , "Tongue of wood for joint" , "Connection" , "Joint - tonne (anag)" , "One projecting" .)