Armed force I illuminated entering target, but retreating (7)
I believe the answer is:
militia
'armed force' is the definition.
(I've seen this before)
'i illuminated entering target but retreating' is the wordplay.
'illuminated' becomes 'lit' (synonyms).
'entering' means one lot of letters goes inside another.
'target' becomes 'aim' (synonyms).
'but retreating' says the letters should be written in reverse (retreat can mean to go back).
'aim' backwards is 'mia'.
'i'+'lit'='ilit'
'ilit' inserted into 'mia' is 'MILITIA'.
(Other definitions for militia that I've seen before include "Group of citizen soldiers" , "Reserve force" , "Armed group" , "Force raised for emergency" , "Armed civilians" .)