Adult butterfly (4)
I believe the answer is:
blue
This clue is a double definition.
'adult' is the first definition.
(I've seen this before)
'butterfly' is the second definition.
(blue is a kind of butterfly)
(Other definitions for blue that I've seen before include "Australian redhead" , "Woodland flower" , "ball at snooker" , "Vulgar" , "Dirty" .)