Abnormal growth
I believe the answer is:
cyst
'abnormal growth' 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 "Biological sac" , "Bag-like growth containing diseased matter" , "Small bladder-like sac" , "Fluid filled sac" , "Blister" .)