Some order van fuel (4)
I believe the answer is:
derv
'fuel' is the definition.
(derv is a kind of fuel)
'some order van' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'DERV' is hidden in the letters of 'order van'.
(Other definitions for derv that I've seen before include "Vehicle diesel-oil" , "Diesel engine fuel oil" , "Lorry fuel" , "Vehicle fuel oil" , "Water monster" .)