Growth under the skin (4)
I believe the answer is:
cyst
'growth under the skin' 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 "Benign growth" , "Bladder" , "Abnormal closed sac in a body" , "Sac in the body containing liquid secretion" , "Small sac in body (that may form abnormal lump)" .)