Group that’s got part of the joint jumping? (5)
I believe the answer is:
nonet
'group' is the definition.
(group of nine musicians)
'part of the joint jumping?' is the wordplay.
'part of the joint' becomes 'tenon' (part of a wooden joint).
'jumping?' is a reversal indicator (in a down clue, letters jump up).
'tenon' in reverse letter order is 'NONET'.
'that's got' is the link.
(Other definitions for nonet that I've seen before include "Piece for group" , "Instrumental group of nine players" , "Ennead" , "Piece for nine musicians" , "Band of nine?" .)