Key to fire engine? (8)
I believe the answer is:
ignition
'fire engine?' is the definition.
'ignition' can be an answer for 'fire' (thesaurus). I'm not certain of the 'engine?' bit.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for ignition that I've seen before include "Starting of combustion" , "The purpose of the spark" , "Process of starting fire" , "Kindling process" , "Point at which combustion occurs" .)