Closed sac
I believe the answer is:
cyst
'closed sac' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cyst that I've seen before include "Fluid filled sac" , "Sac in the body containing liquid secretion" , "Abnormal body cavity" , "Abnormal sac formed on or in the body" , "Abnormal membranous sac" .)