Aid to vision (8)
I believe the answer is:
eyeglass
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">aid to vision</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for eyeglass that I've seen before include "Lens for aiding defective vision" , "Monocle, or spectacles in plural" , "Aid to vision" .)