Colonnade trees (4)
I believe the answer is:
elms
'colonnade trees' 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" , "may have many leaves" , "Mel's strange trees" , "Trees (of late, prone to disease)" , "rugged feature of country churchyard" .)