A fuel (6)
I believe the answer is:
petrol
'a fuel' is the definition.
(I know that petrol is a type of petroleum product)
This is all the clue.
(Other definitions for petrol that I've seen before include "Fuel for the internal combustion engine" , "Common motor fuel" , "Gasoline" , "Fuel for road vehicles" , "Mineral oil refined as fuel" .)