Side with everybody unknown (4)
I believe the answer is:
ally
'side' is the definition.
(thesaurus)
'with everybody unknown' is the wordplay.
'with' is a charade indicator (letters next to each other).
'everybody' becomes 'all' (I've seen this in another clue).
'unknown' becomes 'y' (algebra).
'all'+'y' is 'ALLY'.
(Other definitions for ally that I've seen before include "One in cooperation with another" , "Band together" , "Accomplice" , "Associate that provides assistance" , "State formally cooperating with another" .)