Nightmare with stomach after illness? (7)
I believe the answer is:
bugbear
'nightmare' is the definition.
(thesaurus)
'stomach after illness?' is the wordplay.
'stomach' becomes 'bear' ('bear' can be a synonym of 'stomach').
'after' is a charade indicator (letters next to each other).
'illness?' becomes 'bug'.
'bear' after 'bug' is 'BUGBEAR'.
'with' acts as a link.
(Other definitions for bugbear that I've seen before include "Thing causing obsessive anxiety" , "something terrible" , "Cause of great distress" , "bogeyman" , "pet hate" .)