Joint musical is a slog (5,6)
I believe the answer is:
elbow grease
'a slog' is the definition.
I can't judge whether this defines the answer.
'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'
'is' is the link.
Can you help me to learn more?
(Other definitions for elbow grease that I've seen before include "Hard rubbing (colloq.)" , "Vigorous physical effort" , "Vigorous physical labour involving hard rubbing" , "Hard physical effort" , "Hard physical work (informal)" .)