Biological pouch (3)
I believe the answer is:
sac
'biological pouch' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sac that I've seen before include "Bag, pouch; cyst" , "Bladder" , "Bag-like part in animal or plant" , "Pouch in the body" , "Bag-like part of the body" .)