Ideal notion of this person's past (5)
I believe the answer is:
imago
'ideal notion' is the definition.
(psychoanalytic term)
'this person's past' is the wordplay.
'this person's' becomes 'im' (this person means the crossword setter themselves).
'past' becomes 'ago' ('ago' can be similar in meaning to 'past').
'im'+'ago'='IMAGO'
'of' acts as a link.
(Other definitions for imago that I've seen before include "imagines more than one" , "Comma perhaps" , "Beetle, for example" , "in the developmental stage" , "Sexually mature adult insect" .)