Thick oily substance (6)
I believe the answer is:
grease
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">thick oily substance</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)