Mature insect in yours truly’s past? (5)
I believe the answer is:
imago
'mature insect' is the definition.
(I've seen this before)
'yours truly's past?' is the wordplay.
'yours truly's' becomes 'I'm' (I'm).
'past?' becomes 'ago' ('ago' can be similar in meaning to 'past').
'im'+'ago'='IMAGO'
'in' is the link.
(Other definitions for imago that I've seen before include "Adult insect after metamorphosis" , "Adult stage of an insect, typically winged" , "A mental picture" , "Insect in its adult form" , "Fully-developed insect" .)