Crossword Genius

Something painful on Observer's cover? (4)

Ross

I believe the answer is:

stye

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'cover?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for stye that I've seen before include "Eye sore" , "Infection on the edge of the eye" , "dump" , "Animal pen" , "Climb" .)

I've seen this clue in The Sun.
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