Unusually bad sign for stomach (7)
I believe the answer is:
abdomen
'stomach' is the definition.
(synonyms)
'unusually bad sign' is the wordplay.
'unusually' is an anagram indicator.
'sign' becomes 'omen' (omen is a kind of sign).
'bad' anagrammed gives 'abd'.
'abd'+'omen'='ABDOMEN'
'for' acts as a link.
(Other definitions for abdomen that I've seen before include "Central part of the body" , "pot" , "A main part of the body" , "Insect's body part" , "Part of body containing the digestive organs" .)