Viewer's sore as regularly sat by set? (4)
I believe the answer is:
stye
'viewer's sore' is the definition.
(sore or swelling on an eye or 'viewer')
'regularly sat by set?' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'sat'+'by'+'set'='satbyset'
The alternating letters of 'satbyset' are 'STYE'.
'as' is the link.
(Other definitions for stye that I've seen before include "Eye complaint" , "Eyelid affliction" , "Eye blemish" , "Tamworth residence" , "Infected swelling of the eye" .)