Entirely unknown friend (4)
I believe the answer is:
ally
'friend' is the definition.
('ally' can be a synonym of 'friend')
'entirely unknown' is the wordplay.
'entirely' becomes 'all' (I've seen this before).
'unknown' becomes 'y' (algebra).
'all'+'y'='ALLY'
(Other definitions for ally that I've seen before include "Join forces with" , "Accomplice" , "Friend, partner" , "Friendly nation" , "Band together" .)