Possibly bad sign in the belly (7)
I believe the answer is:
abdomen
'belly' is the definition.
('abdomen' can be a synonym of 'belly')
'possibly bad sign' is the wordplay.
'possibly' indicates anagramming the letters.
'sign' becomes 'omen' (omen is a kind of sign).
'bad' anagrammed gives 'abd'.
'abd'+'omen'='ABDOMEN'
'in the' acts as a link.
(Other definitions for abdomen that I've seen before include "Bodily region" , "Part of body containing the stomach" , "Central part of the body" , "Midriff" , "Where one digests" .)