Trembling tree (5)
I believe the answer is:
aspen
'trembling 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 "city in Colorado" , "One trembling" , "A poplar with shivering leaves" , "Its leaves flutter in the slightest breeze" , "somewhere in Colorado" .)