Pig pen we hear is an eyesore (4)
I believe the answer is:
stye
'is an eyesore' is the definition.
(I've seen this before)
'pig pen we hear' is the wordplay.
'pig pen' becomes 'sty' (where pigs are kept).
'we hear' becomes 'e' (I've seen this in other clues).
'sty'+'e'='STYE'
(Other definitions for stye that I've seen before include "irritation to the viewer" , "Eyelid sore" , "It pains viewer" , "Eye complaint" , "Viewer's complaint" .)