Poplar tree (5)
I believe the answer is:
aspen
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">poplar tree</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that aspen is a type of poplar tree)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for aspen that I've seen before include "One trembling" , "Very timid" , "Tree, kind of poplar" , "somewhere in Colorado" , "that's wooden" .)