National Mall trees (4)
I believe the answer is:
elms
'national mall trees' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for elms that I've seen before include "Trees sound like Cockney steering devices" , "may have many leaves" , "rugged feature of country churchyard" , "Mel's strange trees" , "Trees (of late, prone to disease)" .)