One’s past the final stage (5)
I believe the answer is:
imago
'final stage' is the definition.
(I've seen this before)
'one's past' is the wordplay.
'one's' becomes 'I'm' ('one is' is a formal way of saying 'I am').
'past' becomes 'ago' (similar in meaning).
'im'+'ago'='IMAGO'
'the' acts as a link.
(Other definitions for imago that I've seen before include "Post-pupa stage of an insect" , "idealised figure" , "Maybe comma" , "latest development in cricket?" , "being immature?" .)