Five-sided figure (8)
I believe the answer is:
pentagon
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">five-sided figure</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 pentagon that I've seen before include "Headquarters of the US Defense Department" , "rectilineal figure" , "Figure - US military HQ" , "US military leaders" , "top brass" .)