Petrol (4)
(Other definitions for fuel that I've seen before include "Gas, oil or petrol, for instance" , "Substance to produce energy" , "Stuff to burn" , "Burned source of energy" , "Eg, coal or petrol" .)
'petrol' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for gas that I've seen before include "Oxygen maybe" , "Something exciting" , "Talk idly; fuel" , "Talk profusely" , "Eg, nitrogen" .)