Dirty pen? (3)
I believe the answer is:
sty
'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" .)