Poplar trees (6)
I believe the answer is:
aspens
'poplar trees' is the definition.
(aspen is a kind of poplar tree)
This is the entire clue.
(Other definitions for aspens that I've seen before include "Trees that shiver, kind of poplars" , "Quivering poplars" , "Trees with leaves that quiver" , "Species of poplars with leaves that quiver" , "Trembling poplars" .)