Eight-sided polygon (7)
I believe the answer is:
octagon
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">eight-sided polygon</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 octagon that I've seen before include "Shape of a square with the corners cut off" , "Geometric figure with eight sides" , "Eight-sided figure" , "with eight angles" , "Eight-sided room" .)