Muddy pen (3)
I believe the answer is:
sty
'muddy pen' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sty that I've seen before include "Porcine pen" , "Pig's enclosure" , "Pig-house" , "Swinish place" , "Pig home or eye sore" .)