Fire engine must need this (8)
I believe the answer is:
ignition
'fire' is the definition.
(thesaurus)
'engine must need this' is the wordplay.
I cannot quite see how this works, but
'engine' could be 'ion' (ion engine is a kind of engine) and 'ion' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ignition that I've seen before include "Car engine starter" , "Internal-combustion engine's firing system" , "Act of catching fire" , "Start of combustion" , "Act of starting a fire" .)