Stomach after virus is nightmare (7)
I believe the answer is:
bugbear
'is nightmare' is the definition.
(thesaurus)
'stomach after virus' is the wordplay.
'stomach' becomes 'bear' (both can mean to tolerate or endure).
'after' says to put letters next to each other.
'virus' becomes 'bug'.
'bear' after 'bug' is 'BUGBEAR'.
(Other definitions for bugbear that I've seen before include "Cause of annoyance, fear" , "Bete noire" , "Nuisance" , "Thing causing obsessive anxiety" , "Obsessive idea" .)