Thick oily substance (6)
I believe the answer is:
grease
'thick oily substance' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for grease that I've seen before include "Oily lubricant" , "'Thick, fatty oil (6)'" , "Travolta's 1978 film" , "Possible cause of slip-up" , "bribe" .)