Trembling trees (6)
I believe the answer is:
aspens
'trembling trees' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for aspens that I've seen before include "Poplar trees that shiver in the wind" , "The leaves of the trees flutter in the slightest breeze" , "Trees that shiver, kind of poplars" , "Quivering poplars" , "Species of poplars with leaves that quiver" .)