I'm past the final stage? (5)
I believe the answer is:
imago
'the final stage?' is the definition.
(I've seen this before)
'i'm past' is the wordplay.
'past' becomes 'ago' (similar in meaning).
'im'+'ago'='IMAGO'
(Other definitions for imago that I've seen before include "That's as far as a butterfly can go" , "Insect's adult stage" , "Adult stage of an insect, typically winged" , "Mature insect after metamorphosis" , "Insect's adult phase" .)