Everything unknown to friend (4)
I believe the answer is:
ally
'friend' is the definition.
(an ally is a friend or supporter)
'everything unknown' is the wordplay.
'everything' becomes 'all' (all things).
'unknown' becomes 'y' (algebra).
'all'+'y'='ALLY'
'to' acts as a link.
(Other definitions for ally that I've seen before include "Friend, partner" , "Associate that provides assistance" , "Team up" , "Friend and colleague" , "Friendly nation" .)