Pouch in the body (3)
I believe the answer is:
sac
'pouch in the body' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sac that I've seen before include "Bodily pouch" , "Animal or vegetable pouch" , "Pouch or bag-like structure" , "Distended membrane; pouch-like cavity" , "Membranous cavity" .)