Abnormal sac
I believe the answer is:
cyst
'abnormal 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 "pouch" , "Abnormal closed sac in a body" , "Bag of fluid" , "Thin-walled cavity containing liquid" , "Small bladder-like sac" .)