Fuel rating (6)
I believe the answer is:
octane
'fuel rating' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for octane that I've seen before include "chemical" , "One cat with hydrocarbon compound occurring in petrol" , "Hydrocarbon present in petrol" , "Flammable hydrocarbon - at once (anag)" , "Petrol quality rating" .)