Shade source (3)
(Another definition for elm-tree that I've seen is " Victim of Dutch disease".)
'shade source' 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 "Furniture wood" , "Tree of the genus Ulmus" , "Source of wood" , "Hardwood tree" , "It provides shade" .)
(Other definitions for awning that I've seen before include "Canvas shelter as on shopfront" , "Light protective roof" , "Protective cover" , "Canvas cover over a shop entrance" , "Covering above a window or door" .)