Poplar tree (5)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
aspen
'poplar tree' is the definition.
(aspen is a kind of poplar tree)
This is all the clue.
(Other definitions for aspen that I've seen before include "One trembling" , "Very timid" , "Tree, kind of poplar" , "somewhere in Colorado" , "that's wooden" .)