Pen that's full of oink? (3)
I believe the answer is:
sty
'pen that's full of oink?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sty that I've seen before include "Farmyard enclosure" , "Eye ailment or pig's home" , "Pig house" , "Pig home or eye sore" , "Sore on eyelid" .)