Crossword Genius

Dirty pen? (3)

Ross

I believe the answer is:

sty

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'pen?' is the definition.
(both can mean an animal enclosure)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for sty that I've seen before include "Pig's enclosure" , "Pigpen; swelling" , "Filthy place" , "Pig-pen and eye infection" , "Pig pen" .)

I've seen this clue in the Evening Standard.
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