Gas rating (6)
I believe the answer is:
octane
'gas rating' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for octane that I've seen before include "chemical" , "Ingredient of petroleum" , "A liquid hydrocarbon found in petroleum" , "Flammable hydrocarbon - at once (anag)" , "One cat with hydrocarbon compound occurring in petrol" .)