Eye ailment (4)
I believe the answer is:
stye
'eye ailment' is the definition.
(I've seen this in another clue)
This is the entire 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" .)