Converted plain writing style I'd developed (12)
I believe the answer is:
proselytised
'converted' is the definition.
(I've seen this before)
'plain writing style i'd developed' is the wordplay.
'plain writing' becomes 'prose' (I've seen this in another clue).
'developed' indicates an anagram.
'style'+'id'='styleid'
'styleid' anagrammed gives 'lytised'.
'prose'+'lytised'='PROSELYTISED'
(Other definitions for proselytised that I've seen before include "Tried to convert" , "Converted" .)