Everything unknown to companion (4)
I believe the answer is:
ally
'companion' is the definition.
(thesaurus)
'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 "Accomplice" , "Cooperating country" , "Friend, partner" , "Someone with whom one is in league" , "Colleague, confederate" .)