Gasoline rating (6)
I believe the answer is:
octane
'gasoline 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 "Hydrocarbon present in petrol" , "A liquid hydrocarbon found in petroleum" , "Hydrocarbon found in petrol - at once (anag)" , "Ingredient of petroleum" , "Anti-knock element in petrol" .)