Gas pump rating (6)
I believe the answer is:
octane
'gas pump 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 "One cat with hydrocarbon compound occurring in petrol" , "Fuel component in McLaren vehicle" , "Petrol quality rating" , "Flammable hydrocarbon - at once (anag)" , "Hydrocarbon found in petrol - at once (anag)" .)