"Quaking" tree (5)
I believe the answer is:
'quaking tree' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)