Thick, fatty oil (6)
I believe the answer is:
grease
'thick fatty oil' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for grease that I've seen before include "Fat, film" , "Musical starring John Travolta and Olivia Newton-John" , "Lubricating agent" , "Travolta's 1978 film" , "Mechanic (informal)" .)