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