Belly starts twisting in worrying sign (7)
I believe the answer is:
abdomen
'belly starts' is the definition.
'abdomen' can be an answer for 'belly' (I've seen this before). I'm unsure of the 'starts' bit.
'twisting in worrying sign' is the wordplay.
'twisting' says the letters should be written in reverse (letters twist back).
'in worrying' becomes 'dba' (I can't justify this - if you can you should give a lot more credence to this answer).
'sign' becomes 'omen' (omen is a kind of sign).
'dba' in reverse letter order is 'abd'.
'abd'+'omen'='ABDOMEN'
Can you help me to learn more?
(Other definitions for abdomen that I've seen before include "Part of body containing the digestive organs" , "corporation's site" , "Central region" , "pot" , "Midriff" .)