Foe's opposite (4)
(Other definitions for ally that I've seen before include "Supporter in a conflict" , "Friend, partner" , "State formally cooperating with another" , "Join - friend" , "Co-operating person or nation" .)
(Other definitions for friend that I've seen before include "Pal or buddy" , "Close associate" , "Associate, supporter" , "one quaking in society?" , "Ally, supporter" .)