Musical group that gets the joint jumping (5)
I believe the answer is:
nonet
'musical group' is the definition.
(group of nine musicians)
'the joint jumping' is the wordplay.
'the joint' becomes 'tenon' (I've seen this in other clues).
'jumping' says the letters should be written backwards (in a down clue, letters jump up).
'tenon' in reverse letter order is 'NONET'.
'that gets' acts as a link.
(Other definitions for nonet that I've seen before include "Ennead" , "Musical group" , "Piece for group" , "piece to be performed" , "Work for nine players" .)