Civilian fighters lost in battle getting one fired (7)
I believe the answer is:
militia
'civilian fighters lost' is the definition.
I can't tell whether this definition defines the answer.
'battle getting one fired' is the wordplay.
'battle' becomes 'mia' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'getting' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'fired' becomes 'lit' (set on fire).
'i'+'lit'='ilit'
'mia' enclosing 'ilit' is 'MILITIA'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for militia that I've seen before include "Group of citizen soldiers" , "Fighting force of civilians" , "Citizen soldiery" , "Civilian soldiery" , "levy" .)