Writer in overpoetic beginning (6)
I believe the answer is:
opener
'beginning' is the definition.
(I know that opener is a type of start)
'writer in overpoetic' is the wordplay.
'writer' becomes 'pen' (something which writes).
'in' is an insertion indicator.
'overpoetic' becomes 'oer' (I am not sure about this - if you are sure you should believe this answer much more).
'pen' put inside 'oer' is 'OPENER'.
Can you help me to learn more?
(Other definitions for opener that I've seen before include "cricketer receiving the first ball?" , "First part" , "device for getting at the drink" , "Batsman number one" , "First event in a series" .)