Mature insect, one initially misidentified in the past (5)
I believe the answer is:
imago
'mature insect' is the definition.
(I've seen this before)
'one initially misidentified in the past' is the wordplay.
'one' becomes 'i' (Roman numeral).
'initially' says to take the initial letters.
'in the past' becomes 'ago' ('ago' can be a synonym of 'in the past').
The initial letter of 'misidentified' is 'm'.
'i'+'m'+'ago'='IMAGO'
(Other definitions for imago that I've seen before include "Mature insect stage" , "spinner, say" , "Insect at end of life" , "subconscious view" , "Final stage of an insect's development" .)