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It shows the quality of petrol at once, possibly (6)

Ross

I believe the answer is:

octane

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'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)" .)

I've seen this clue in The Globe and Mail.
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