Crossword Genius

Petrol has this running at once (6)

Ross

I believe the answer is:

octane

'petrol has this' is the definition.
(octane is contained in petroleum)

'running at once' is the wordplay.
'running' indicates an anagram.
'at'+'once'='atonce'
'atonce' is an anagram of 'OCTANE'.

(Other definitions for octane that I've seen before include "Ingredient of petroleum" , "Anti-knock rating" , "A liquid hydrocarbon found in petroleum" , "chemical" , "Petrol quality rating" .)

I've seen this clue in The Times.
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