Optical illusion on a butterfly's wing? (7)
I believe the answer is:
eyespot
'on a butterfly's wing?' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for eyespot that I've seen before include "Evidence of disease" , "Plant disease" , "Light-sensitive patch on a starfish and others" , "Cereal fungal disease" , "on a butterfly's wing" .)