Antithetical (8)
I believe the answer is:
opposite
'antithetical' is the definition.
(synonyms)
This is all the clue.
(Other definitions for opposite that I've seen before include "Antithetical, vis-a-vis at" , "Reverse or antithesis" , "Contrary or face-to-face" , "Adverse" , "Directly facing each other" .)