Diesel oil (4)
I believe the answer is:
derv
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">diesel oil</span>' is the definition.<br/ ><span class="explanation-format__explanation">(derv is a kind of diesel oil)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for derv that I've seen before include "Water monster" , "Lorry fuel" , "Vehicle fuel oil" , "Type of fuel" , "Fuel for lorries" .)