Stomach terribly bad sign (7)
I believe the answer is:
abdomen
'stomach' is the definition.
(synonyms)
'terribly bad sign' is the wordplay.
'terribly' indicates anagramming the letters.
'sign' becomes 'omen' (omen is a kind of sign).
'bad' with letters rearranged gives 'abd'.
'abd'+'omen'='ABDOMEN'
(Other definitions for abdomen that I've seen before include "Ban demo (anag)" , "Central part of the body" , "Belly" , "gut" , "Tummy" .)