Trees since found by writers (6)
I believe the answer is:
aspens
'trees' is the definition.
(aspen is a kind of tree)
'since found by writers' is the wordplay.
'since' becomes 'as' (both can mean 'because').
'found by' says to put letters next to each other.
'writers' becomes 'pens' (writing implements).
'as'+'pens'='ASPENS'
(Other definitions for aspens that I've seen before include "Trees that shiver, kind of poplars" , "Trembling poplars" , "Kind of poplars with shivering leaves" , "Trembling trees" , "Poplar trees that shiver in the wind" .)