Completely unknown bedfellow (4)
I believe the answer is:
ally
'bedfellow' is the definition.
(thesaurus)
'completely unknown' is the wordplay.
'completely' becomes 'all' ('all' can be a synonym of 'completely').
'unknown' becomes 'y' (algebra).
'all'+'y'='ALLY'
(Other definitions for ally that I've seen before include "Crony" , "Friend and colleague" , "Supporter in a conflict" , "Join by marriage, treaty etc" , "support" .)