A lubricant (6)
I believe the answer is:
grease
'a lubricant' 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 "Musical animal fat?" , "Musical starring John Travolta and Olivia Newton-John" , "Lubricating agent" , "Musical that sounds like a country" , "'Thick, fatty oil (6)'" .)