Musical lubricant (6)
I believe the answer is:
grease
I believe this is a double definition.
'musical' is the first definition.
(Grease is a film musical)
'lubricant' is the second definition.
(I've seen this before)
(Other definitions for grease that I've seen before include "Lubricate - musical" , "Rock musical" , "Possible cause of slip-up" , "Lubricate (-ant)" , "Fatty lubricant" .)