What irritated viewer during last year? (4)
I believe the answer is:
stye
'what irritated viewer' is the definition.
(infection in an eye or 'viewer')
'during last year?' is the wordplay.
'during' indicates the answer is hidden within the clue.
'STYE' is hidden within 'last year'.
(Other definitions for stye that I've seen before include "Red eye" , "Eyelid problem" , "Swelling near the eye" , "Climb" , "pain for the observer" .)