Completely unknown supporter (4)
I believe the answer is:
ally
'supporter' is the definition.
(ally can mean a supporter or friend)
'completely unknown' is the wordplay.
'completely' becomes 'all' (synonyms).
'unknown' becomes 'y' (algebra).
'all'+'y'='ALLY'
(Other definitions for ally that I've seen before include "Associate that provides assistance" , "Cooperating country" , "A friendly and reliable country" , "Supporter in a conflict" , "Friend and colleague" .)