Radical writer taking arsenic (5)
I believe the answer is:
basic
'radical' is the definition.
(similar in meaning)
'writer taking arsenic' is the wordplay.
'writer' becomes 'bic' (kind of pen).
'taking' indicates putting letters inside.
'arsenic' becomes 'As' (As is the chemical symbol for arsenic).
'bic' going around 'as' is 'BASIC'.
(Other definitions for basic that I've seen before include "Fundamental -- early computer language" , "Elemental" , "Fundamental; alkaline" , "Fundamental, forming a starting-point" , "Elementary - without extras" .)