Everything unknown to friend (4)
I believe the answer is:
Here is my best explanation:
'friend' is the definition.
('ally' can be a synonym of 'friend')
'everything unknown' is the subsidiary indication.
'everything' becomes 'all' (I've seen this before).
'unknown' becomes 'y' (algebra).
'to' is the link.
(Other definitions for ally that I've seen before include "One on one's side", "Friend and colleague", "Team up", "Colleague, confederate", "Cooperating country", "Cooperating state", "Co-operating person or nation", "A friendly nation", "Co-operative friend" and "Associate that provides assistance".)