Writer's accepted as essential (5)
I believe the answer is:
basic
'essential' is the definition.
('basic' can be similar in meaning to 'essential')
'writer's accepted as' is the wordplay.
'writer' becomes 'bic' (kind of pen).
'accepted' means one lot of letters goes inside another.
'bic' going around 'as' is 'BASIC'.
(Other definitions for basic that I've seen before include "Without frills" , "Fundamental -- early computer language" , "Plain" , "Popular programming language" , "Fundamental, elementary" .)