Viral spread for opponent (5)
I believe the answer is:
rival
'for opponent' is the definition.
(a rival is an opponent or enemy)
'viral spread' is the wordplay.
'spread' indicates anagramming the letters.
'viral' with letters rearranged gives 'RIVAL'.
(Other definitions for rival that I've seen before include "Adversary or foe" , "Compete with" , "Person in direct competition" , "Close competitor" , "Challenger, competitor" .)