Tummy upset? Bad sign! (7)
I believe the answer is:
abdomen
'tummy' is the definition.
(I've seen this before)
'upset? bad sign' is the wordplay.
'upset?' indicates anagramming the letters (I've seen 'upset' mean this).
'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 "Belly" , "Tummy" , "A main part of the body" , "Region of the body between the thorax and pelvis" , "Site of corporation" .)