Friend may be completely unknown (4)
I believe the answer is:
ally
'friend' is the definition.
(an ally is a friend or supporter)
'completely unknown' is the wordplay.
'completely' becomes 'all' (synonyms).
'unknown' becomes 'y' (algebra).
'all'+'y'='ALLY'
'may be' is the link.
(Other definitions for ally that I've seen before include "Confederate; supporter" , "Join by marriage, treaty etc" , "Cooperating country" , "mate" , "Crony" .)