Volatile substance used at once (6)
I believe the answer is:
octane
'volatile substance' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'used at once' is the wordplay.
'used' 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 "Hydrocarbon present in petrol" , "Flammable hydrocarbon - at once (anag)" , "One cat with hydrocarbon compound occurring in petrol" , "Fuel component in McLaren vehicle" , "Anti-knock rating" .)