London tree? (6)
I believe the answer is:
This clue might be a double definition.
'london' is the first definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'tree?' is the second definition.
(poplar is a kind of tree)
Can you help me to learn more?
(Other definitions for poplar that I've seen before include "Leaves producer" , "Tall tree of north temperate regions" , "The tree is almost liked" , "Slender tree with quivering leaves" , "Tree of willow family" .)