National Mall tree (3)
I believe the answer is:
elm
'national mall tree' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for elm that I've seen before include "Victim of fungus" , "Stately tree" , "Gray's rugged tree (Elegy)" , "Tree for a Cockney tiller" , "Tall shade tree" .)