Simple when written in pen (5)
I believe the answer is:
basic
'simple' is the definition.
(synonyms)
'when written in pen' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'written in' indicates putting letters inside.
'pen' becomes 'bic' (pen manufacturer).
'as' placed inside 'bic' is 'BASIC'.
(Other definitions for basic that I've seen before include "Fundamental -- early computer language" , "Fundamental; alkaline" , "Fundamental, forming a starting-point" , "Introductory" , "'Primary, fundamental (5)'" .)