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' (I've seen this in other clues).
'found by' means one lot of letters go next to another.
'writers' becomes 'pens' (I've seen this in another clue).
'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" .)