Squid's ink holder (3)
I believe the answer is:
sac
'squid's ink holder' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sac that I've seen before include "Pouch in the body" , "Bag-like part of the body" , "Traditional entitlement to hold court" , "Distended membrane; pouch-like cavity" , "Bag-like part in animal or plant" .)