Girl sheltering resistance fighters (4)
I believe the answer is:
army
'fighters' is the definition.
(armies fight)
'girl sheltering resistance' is the wordplay.
'girl' becomes 'amy' ().
'sheltering' indicates putting letters inside.
'resistance' becomes 'r' (electronics abbreviation).
'amy' going around 'r' is 'ARMY'.
(Other definitions for army that I've seen before include "Throng" , "Horde" , "Body of people organised for war" , "Large number (of ants?)" , "A great number (of soldiers?)" .)