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 wordplay.
'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 "Collaborator, affiliate", "Friendly nation", "Crony", "Friend and colleague", "Comrade in arms", "Join - friend", "Team up", "Someone with whom one is in league", "Co-operative friend" and "Colleague, confederate".)