Thick lubricant (6)
I believe the answer is:
grease
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">thick lubricant</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 "Lubricate (-ant)" , "Musical starring John Travolta and Olivia Newton-John" , "Lubricating agent" , "A thick oily substance" , "Fat, film" .)