Foe has viral problem (5)
I believe the answer is:
rival
'foe' is the definition.
(I've seen this before)
'viral problem' is the wordplay.
'problem' indicates anagramming the letters.
'viral' is an anagram of 'RIVAL'.
'has' acts as a link.
(Other definitions for rival that I've seen before include "Contender" , "Adversary, antagonist" , "Challenger, competitor" , "Close competitor" , "Be equal to in quality" .)