Guts churning - bad sign (7)
I believe the answer is:
abdomen
'guts' is the definition.
(I've seen this before)
'churning bad sign' is the wordplay.
'churning' indicates an anagram.
'sign' becomes 'omen' (omen is a kind of sign).
'bad' is an anagram of 'abd'.
'abd'+'omen'='ABDOMEN'
(Other definitions for abdomen that I've seen before include "Central part of the body" , "Part of body containing the digestive organs" , "guts" , "Part of body containing the stomach" , "Site of corporation" .)