Terrible thing in stomach after virus (7)
I believe the answer is:
bugbear
'terrible thing' is the definition.
(a bugbear is something someone finds annoying)
'stomach after virus' is the wordplay.
'stomach' becomes 'bear' (both can mean to tolerate or endure).
'after' means one lot of letters go next to another.
'virus' becomes 'bug'.
'bear' after 'bug' is 'BUGBEAR'.
'in' is the link.
(Other definitions for bugbear that I've seen before include "One's special fear" , "Thing causing obsessive anxiety" , "something terrible" , "A source of annoyance" , "Nasty goblin - cause of fear" .)