Insect in its adult form (5)
I believe the answer is:
imago
'insect in its adult form' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for imago that I've seen before include "Sexually mature adult insect" , "Last stage of adult insect" , "Maybe comma" , "Comma perhaps" , "Fully-developed insect" .)