Adult stage in insects (5)
I believe the answer is:
imago
'adult stage in insects' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for imago that I've seen before include "For example, monarch ultimately" , "Comma perhaps" , "Mature insect stage" , "Last stage of insect's development" , "Final stage of an insect's development" .)