Feel modest about tree (3)
I believe the answer is:
elm
'tree' is the definition.
(elm is a kind of tree)
'feel modest about' is the wordplay.
'about' says the answer is hidden in the clue.
'ELM' can be found hidden inside 'feel modest'.
(Other definitions for elm that I've seen before include "Source of wood" , "Timber tree" , "Gray's rugged tree (Elegy)" , "Mel's tree" , "Forest tree" .)