Petrol has this running at once (6)
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" .)