Opponent from Byzantium (4)
I believe the answer is:
anti
'opponent' is the definition.
(I've seen this before)
'from byzantium' is the wordplay.
'from' indicates the answer is hidden within the clue.
'ANTI' is hidden in the letters of 'byzantium'.
(Other definitions for anti that I've seen before include "Prefix meaniing against" , "Person opposed" , "Someone opposed (to a policy?)" , "Against, informally" , "Opposed to followers of Hitler" .)