Animal enclosure (3)
I think it could be either:
'animal enclosure' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pen that I've seen before include "Small enclosure" , "Coop" , "Female swan" , "International Association of poets, playwrights etc" , "'The ... is mightier than the sword'" .)
'animal enclosure' is the definition.
(pig sty)
This is all the clue.
(Other definitions for sty that I've seen before include "Swinish place" , "Pig house" , "Farmyard enclosure" , "Hog house" , "Animal pen" .)