"Quaking" tree (5)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">quaking tree</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for aspen that I've seen before include "winter resort" , "Tree that quivers" , "Type of poplar" , "quill-like?" , "The tree that shivers in the slightest wind" .)