Unwanted optical effects (5)
I believe the answer is:
styes
'unwanted optical effects' is the definition.
Although both the answer and definition are plural nouns, I don't understand how they can define each other.
This is all the clue.
Can you help me to learn more?
(Other definitions for styes that I've seen before include "They make it difficult to see" , "Pig pens" , "gatherings" , "Eyelid swellings" , "Eye infections" .)