Simple, when covered by writer (5)
I believe the answer is:
basic
'simple' is the definition.
('basic' can be similar in meaning to 'simple')
'when covered by writer' is the wordplay.
'when' becomes 'as'.
'covered by' means one lot of letters goes inside another.
'writer' becomes 'bic' (kind of pen).
'as' going inside 'bic' is 'BASIC'.
(Other definitions for basic that I've seen before include "Plain" , "Simple form of language" , "Radical" , "Rudimentary - programming language" , "Simple, unadorned" .)