Viewer, part of the year (3)
I believe the answer is:
eye
'viewer' is the definition.
(I've seen this before)
'part of the year' is the wordplay.
'part of' indicates a hidden word.
'EYE' is hidden in the letters of 'the year'.
(Other definitions for eye that I've seen before include "Keep watch on" , "Light-detector" , "Viewer" , "View" , "Look at with interest" .)