Filling station number (6)
I believe the answer is:
octane
'filling station number' 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 "Petrol quality rating" , "Hydrocarbon found in petrol - at once (anag)" , "Fuel component in McLaren vehicle" , "A liquid hydrocarbon found in petroleum" , "One cat with hydrocarbon compound occurring in petrol" .)