Last stage of insect development (5)
I believe the answer is:
imago
'last stage of insect development' 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 "being immature?" , "Winged insect -- amigo (anag)" , "Last stage of insect's development" , "Beetle, for example" , "Adult insect after metamorphosis" .)