Thoroughfare you once followed is an eyesore (4)
I believe the answer is:
stye
'is an eyesore' is the definition.
(I've seen this before)
'thoroughfare you once followed' is the wordplay.
'thoroughfare' becomes 'st' (I've seen this in other clues).
'you once' becomes 'ye' (archaic version of 'you').
'followed' says to put letters next to each other (some letters go after others).
'st'+'ye'='STYE'
(Other definitions for stye that I've seen before include "swelling by organ" , "nasty spot" , "It pains viewer" , "Eyelid affliction" , "Inflammation on eyelid" .)