Text originator's certificate, cornier western included (12)
I believe the answer is:
scriptwriter
'text originator's certificate cornier western included' is the definition.
I can't tell whether this definition defines the answer.
'certificate cornier western included' is the wordplay.
'certificate' becomes 'scrip' (scrip is a kind of certificate).
'cornier' becomes 'triter' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'western' becomes 'w'.
'included' means one lot of letters goes inside another.
'triter' going around 'w' is 'twriter'.
'scrip'+'twriter'='SCRIPTWRITER'
Can you help me to learn more?
(Other definitions for scriptwriter that I've seen before include "One producing TV drama texts" , "Author of words spoken by actors, in play, film etc" , "One writing for films" , "Author of broadcast text" , "Cinema author" .)