Partisan (3-5)
I believe the answer is:
one-sided
'partisan' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for one-sided that I've seen before include "So denied being unfair or biased" , "Indeed so (anag.)" , "Grossly unequal; biased" , "Unequal like contest between badly matched opponents" , "Unbalanced" .)