Gas pump number (6)
I believe the answer is:
octane
'gas pump number' 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 "One cat (anag.)" , "Ingredient of petroleum" , "Anti-knock rating" , "Petrol quality rating" , "Hydrocarbon found in petrol - at once (anag)" .)