Ninety-degree corner (5,5)
I believe the answer is:
right angle
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">ninety-degree corner</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for right angle that I've seen before include "Part of a square" , "that's formed by 1 Across and Down" , "Square's corner" , "Ninety degree corner" , "a good number of degrees?" .)