Pigpen, we hear, is an eyesore? (4)
I believe the answer is:
stye
'is an eyesore?' is the definition.
(I've seen this before)
'pigpen we hear' is the wordplay.
'pigpen' becomes 'sty' ('sty' can be a synonym of 'pigpen').
'we hear' indicates a 'sounds like' (homophone) clue.
'sty' sounds like 'STYE'.
(Other definitions for stye that I've seen before include "producing eyesore" , "Inflammation on eyelid" , "Tamworth residence" , "Ocular inflammation" , "Eyelid affliction" .)