Dermatological sac
I believe the answer is:
cyst
'dermatological sac' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cyst that I've seen before include "Abnormal membranous sac" , "Closed sac in the body, containing fluid" , "Sac or vesicle" , "Fluid-filled sac (med.)" , "Blister" .)