Everything unknown to a friend (4)
I believe the answer is:
ally
'to a 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'
(Other definitions for ally that I've seen before include "Cooperating country" , "Co-operating person or nation" , "One in cooperation with another" , "Band together" , "Friend in time of need" .)