Singular writer behind a tree (5)
I believe the answer is:
aspen
'tree' is the definition.
(aspen is a kind of tree)
'singular writer behind a' is the wordplay.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'writer' becomes 'pen' (something which writes).
'behind' means one lot of letters go next to another.
's'+'pen'='spen'
'spen' after 'a' is 'ASPEN'.
(Other definitions for aspen that I've seen before include "Tremulous poplar" , "winter resort" , "Trembling poplar tree" , "quill-like?" , "Panes (anag.)" .)