Vesicle (4)
I believe the answer is:
cyst
'vesicle' is the definition.
(I know that vesicle can be written as cyst)
This is all the clue.
(Other definitions for cyst that I've seen before include "Bladder" , "Abnormal sac formed on or in the body" , "Blister" , "Biological sac" , "Fluid filled sac" .)