Eye ailment (3)
'eye ailment' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for stye that I've seen before include "Inflamed swelling on the edge of an eyelid" , "Observer's problem" , "Red eye" , "Infection on the edge of the eye" , "Seeing problem" .)
(Other definitions for sty that I've seen before include "Pig-house" , "Enclosure for swine" , "Pig home or eye sore" , "Pig's enclosure" , "Pig pen" .)