Joint musical is hard work (5,6)
I believe the answer is:
elbow grease
'is hard work' is the definition.
(I've seen this in another clue)
'joint musical' is the wordplay.
'joint' becomes 'elbow joint' (joint of the body).
'musical' becomes 'grease' (Grease is a film musical).
'elbow'+'grease'='ELBOW-GREASE'
(Other definitions for elbow grease that I've seen before include "Hard rubbing (colloq.)" , "Hard physical work (informal)" , "Hard physical effort" , "Vigorous physical effort" , "Vigorous physical labour involving hard rubbing" .)