Bitter outstanding resistance meeting writer (7)
I believe the answer is:
acerbic
'bitter' is the definition.
('acerbic' can be a synonym of 'bitter')
'outstanding resistance meeting writer' is the wordplay.
'outstanding' becomes 'ace' (I've seen this before).
'resistance' becomes 'r' (abbreviation).
'meeting' means one lot of letters go next to another.
'writer' becomes 'bic' (kind of pen).
'ace'+'r'+'bic'='ACERBIC'
(Other definitions for acerbic that I've seen before include "Acrimonious" , "Cutting" , "Tasting sour or caustic in speech" , "Harsh and sharp in speech" , "Astringent in tone" .)