Form of fuel (4)
I believe the answer is:
coke
'form of fuel' is the definition.
(I know that coke is a type of fuel)
This is all the clue.
(Other definitions for coke that I've seen before include "Solid fuel - drink - drug" , "Residue from coal used as fuel" , "Substance left after gas and tar have been extracted from coal" , "Fuel derived from coal" , "'Soft drink and hard drug, in short (4)'" .)