A bump in infancy's traumatic (4)
I believe the answer is:
cyst
'a bump' is the definition.
(thesaurus)
'in infancy's traumatic' is the wordplay.
'in' says the answer is hidden in the clue.
'CYST' is hidden in the letters of 'infancys traumatic'.
(Other definitions for cyst that I've seen before include "Small sac in body (that may form abnormal lump)" , "Sac in the body containing liquid secretion" , "Bag of fluid" , "Biological sac" , "Sac, growth" .)