That London tree (6)

I believe the answer is:

poplar

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'tree' is the definition.
(poplar is a kind of tree)

I don't understand the rest of the clue.

(Other definitions for poplar that I've seen before include "Tall tree of north temperate regions" , "Deciduous tree" , "Tree, maybe from Lombardy" , "tree found mostly in the northern hemisphere" , "Tree - London district" .)

I've seen this clue in the Evening Standard.

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