Enemy has viral problem (5)
I believe the answer is:
rival
'enemy' is the definition.
(thesaurus)
'viral problem' is the wordplay.
'problem' indicates an anagram.
'viral' is an anagram of 'RIVAL'.
'has' acts as a link.
(Other definitions for rival that I've seen before include "Contestant you hope to defeat" , "Compete with" , "Contender" , "Someone you hope to defeat" , "Emulate" .)