Engine fuel (6)
I believe the answer is:
petrol
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">engine fuel</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 petrol that I've seen before include "Common car fuel" , "The juice" , "Fuel for road vehicles" , "Gasoline" , "Common motor fuel" .)