Stomach terribly bad portent (7)
I believe the answer is:
abdomen
'stomach' is the definition.
('abdomen' can be a synonym of 'stomach')
'terribly bad portent' is the wordplay.
'terribly' indicates anagramming the letters.
'portent' becomes 'omen' ('omen' can be a synonym of 'portent').
'bad' with letters rearranged gives 'abd'.
'abd'+'omen'='ABDOMEN'
(Other definitions for abdomen that I've seen before include "Tummy" , "Mean bod (anag.)" , "Bodily region" , "Site of corporation" , "Ban demo (anag)" .)