Growth in helmet (3)
I believe the answer is:
elm
'growth' is the definition.
'in helmet' is the wordplay.
'in' says the answer is hidden in the clue.
'ELM' is hidden in the letters of 'helmet'.
(Other definitions for elm that I've seen before include "Mel's tree" , "Source of timber" , "Timber tree" , "Deciduous tree with rough serrated leaves" , "Tree of the genus Ulmus" .)