A lubricant (6)
I believe the answer is:
grease
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">a lubricant</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for grease that I've seen before include "Musical animal fat?" , "Musical starring John Travolta and Olivia Newton-John" , "Lubricating agent" , "Musical that sounds like a country" , "'Thick, fatty oil (6)'" .)