Sight defect (6,6)
I believe the answer is:
tunnel vision
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sight defect</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 tunnel vision that I've seen before include "Seeing problem" , "Narrowness of viewpoint" , "unwavering concentration? - with 4a" , "dream Napoleon had for invading England?" , "Condition where one can only see objects directly ahead" .)