Bag-like structure (3)
I believe the answer is:
sac
'bag-like structure' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sac that I've seen before include "Traditional entitlement to hold court" , "Bag-like cavity" , "Pouch or bag-like structure" , "Bladder" , "Membranous cavity" .)