"Summer Nights" musical (6)
I believe the answer is:
grease
'summer nights musical' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for grease that I've seen before include "Fat, film" , "Musical animal fat?" , "'Thick, fatty oil (6)'" , "-- gun; -- monkey" , "Musical - lubricant" .)