Thick lubricant (6)
I believe the answer is:
grease
'thick lubricant' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for grease that I've seen before include "Lubricate (-ant)" , "Musical starring John Travolta and Olivia Newton-John" , "Lubricating agent" , "A thick oily substance" , "Fat, film" .)