Poplar cousins (6)
I believe the answer is:
aspens
'poplar cousins' 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 "Kind of poplars with shivering leaves" , "Trees that shiver, kind of poplars" , "Poplar trees that shiver in the wind" , "Trembling poplars" , "Trees with leaves that quiver" .)