Tug back on stomach (3)
I believe the answer is:
gut
'stomach' is the definition.
(I've seen this in other clues)
'tug back' is the wordplay.
'back' shows that the letters should be reversed in order.
'tug' written backwards gives 'GUT'.
'on' is the link.
(Other definitions for gut that I've seen before include "Stomach, belly" , "Thread made from animal intestines" , "spare tyre" , "Material for violins" , "Intestine, or the stomach" .)