Poplar (5)
I believe the answer is:
aspen
'poplar' is the definition.
(I know that aspen is a type of poplar)
This is all the clue.
(Other definitions for aspen that I've seen before include "Tree that quivers" , "Colorado ski resort - tree" , "The tree that shivers in the slightest wind" , "quill-like?" , "One trembling" .)