Tree; London district (6)
I believe the answer is:
poplar
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">tree</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(poplar is a kind of tree)</span><br/ ><br/ >'<span class="explanation-format__original">london district</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(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" .)