This writer's past an idealised representation (5)
I believe the answer is:
imago
'an idealised representation' is the definition.
(psychological term)
'this writer's past' is the wordplay.
'this writer's' becomes 'i'm' ('I am' referring to the writer of the clue).
'past' becomes 'ago' (similar in meaning).
'im'+'ago'='IMAGO'
(Other definitions for imago that I've seen before include "imagines more than one" , "Adult insect after metamorphosis" , "Picture of saint" , "A mental picture" , "spinner, say" .)