Farm building reportedly an eyesore (4)
I believe the answer is:
stye
'eyesore' is the definition.
(I've seen this before)
'farm building reportedly' is the wordplay.
'farm building' becomes 'sty'.
'reportedly' indicates a 'sounds like' (homophone) clue.
'sty' is a homophone of 'STYE'.
'an' acts as a link.
(Other definitions for stye that I've seen before include "swelling by organ" , "It pains viewer" , "Inflammation on eyelid" , "nasty spot" , "Ocular inflammation" .)