Part of the digestive system (3)
I believe the answer is:
gut
'part of the digestive system' is the definition.
('gut' is part of 'digestive system')
This is the entire clue.
(Other definitions for gut that I've seen before include "corporation" , "Material for violins" , "Intestine, or the stomach" , "restricted passage" , "Intestine; narrow strait" .)