Type of fuel (6)
I believe the answer is:
diesel
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">type of fuel</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 diesel that I've seen before include "Vehicle fuel" , "Derv" , "Rudolf __ , French-born German engine inventor, d.1913" , "Liquid fuel" , "Common fuel for heavy vehicles" .)