Horribly bad sign for belly (7)
I believe the answer is:
abdomen
'for belly' is the definition.
(synonyms)
'horribly bad sign' is the wordplay.
'horribly' indicates an anagram (a horribly wrong version of the letters).
'sign' becomes 'omen' (omen is a kind of sign).
'bad' anagrammed gives 'abd'.
'abd'+'omen'='ABDOMEN'
(Other definitions for abdomen that I've seen before include "Mean bod (anag.)" , "Tummy" , "Bodily region" , "Central part of the body" , "Site of corporation" .)