Quaking trees (6)
I believe the answer is:
aspens
'quaking trees' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for aspens that I've seen before include "The leaves of the trees flutter in the slightest breeze" , "Trembling trees" , "Quivering poplars" , "Kind of poplars with shivering leaves" , "Species of poplars with leaves that quiver" .)