Force aim on way back to seize leaders of insurgents left in town (7)
I believe the answer is:
militia
'force' is the definition.
(I know that militia is a type of force)
'aim on way back to seize leaders of insurgents left in town' is the wordplay.
'on way back' shows that the letters should be reversed in order.
'to seize' indicates putting letters inside.
'leaders of' suggests taking the first letters.
The first letters of 'insurgents left in town' is 'ilit'.
'aim' reversed gives 'mia'.
'mia' enclosing 'ilit' is 'MILITIA'.
(Other definitions for militia that I've seen before include "Armed civilians" , "Military force of trained civilians" , "Civilian soldiery" , "Home Guard" , "Troops" .)