Tree; London district (6)
I believe the answer is:
poplar
I believe this clue is a double definition.
'tree' is the first definition.
(poplar is a kind of tree)
'london district' is the second definition.
(I've seen this before)
(Other definitions for poplar that I've seen before include "Tree, maybe from Lombardy" , "Tree with leaves that quiver" , "Tree of north temperate regions, with flowers borne in catkins" , "Tall tree (which may tremble?)" , "Tree of willow family" .)