Bitter writer sitting beneath tree (7)
I believe the answer is:
acerbic
'bitter' is the definition.
(synonyms)
'writer sitting beneath tree' is the wordplay.
'writer' becomes 'bic' (kind of pen).
'sitting beneath' says to put letters next to each other (in a down clue, some letters go underneath others).
'tree' becomes 'acer' (I've seen this before).
'bic' after 'acer' is 'ACERBIC'.
(Other definitions for acerbic that I've seen before include "Acrimonious" , "Biting" , "Tasting bitter; forthright" , "Sour; sharp" , "Harsh and sharp in speech" .)