Growth in infancy, steady (4)
I believe the answer is:
cyst
'growth' is the definition.
(I've seen this before)
'in infancy steady' is the wordplay.
'in' indicates a hidden word.
'CYST' is hidden within 'infancy steady'.
(Other definitions for cyst that I've seen before include "Sac or vesicle" , "Abnormal membranous sac" , "Fluid filled sac" , "Membrane" , "Small sac in body (that may form abnormal lump)" .)