Feel modest about growth (3)
I believe the answer is:
elm
'growth' is the definition.
'feel modest about' is the wordplay.
'about' indicates the answer is hidden within the clue (the clue goes around or about the answer).
'ELM' can be found hidden inside 'feel modest'.
(Other definitions for elm that I've seen before include "Tree of the genus Ulmus" , "Species of tree" , "Tree for a Cockney tiller" , "Source of wood" , "Tree with winged fruit" .)