Explosion of octane now (2,4)

I believe the answer is:

at once

'now' is the definition.
(thesaurus)

'explosion of octane' is the wordplay.
'explosion of' is an anagram indicator.
'octane' anagrammed gives 'AT ONCE'.

(Other definitions for at once that I've seen before include "without hesitation" , "'Immediately (2,4)'" , "Octane (anag)" , "This minute!" , "Immediately, without delay" .)

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.