It shows the quality of petrol at once, possibly (6)
I believe the answer is:
octane
'it shows the quality of petrol' is the definition.
The definition suggests a singular noun which matches the answer.
'at once possibly' is the wordplay.
'possibly' indicates an anagram.
'at'+'once'='atonce'
'atonce' is an anagram of 'OCTANE'.
Can you help me to learn more?
(Other definitions for octane that I've seen before include "One cat with hydrocarbon compound occurring in petrol" , "Hydrocarbon present in petrol" , "chemical" , "Ingredient of petroleum" , "Hydrocarbon found in petrol - at once (anag)" .)