This writer is past final stage in development (5)
I believe the answer is:
imago
'stage in development' is the definition.
'imago' can be an answer for 'stage' (I have seen 'mature stage' mean 'imago' so perhaps 'stage' could also mean 'imago'). I am not sure about the 'in development' bit.
'this writer is past final' is the wordplay.
'this writer is' becomes 'I'm' (I am - referring to the crossword setter).
'past' becomes 'ago' (similar in meaning).
'final' says to put letters next to each other.
'im'+'ago'='IMAGO'
Can you help me to learn more?
(Other definitions for imago that I've seen before include "Last stage of adult insect" , "Comma perhaps" , "spinner, say" , "Picture of saint" , "Post-pupa stage of an insect" .)