Terribly bad sign in the middle (7)
I believe the answer is:
abdomen
'the middle' is the definition.
(thesaurus)
'terribly bad sign' is the wordplay.
'terribly' is an anagram indicator.
'sign' becomes 'omen' (omen is a kind of sign).
'bad' is an anagram of 'abd'.
'abd'+'omen'='ABDOMEN'
'in' is the link.
(Other definitions for abdomen that I've seen before include "Belly" , "Part of body containing the stomach" , "4 part of insect" , "gut" , "guts" .)