Adult insect stage (5)
I believe the answer is:
imago
'adult insect stage' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for imago that I've seen before include "picture of important person" , "For example, monarch ultimately" , "Fully-developed insect" , "Insect's adult phase" , "Sexually mature adult insect" .)