Side of a cut gem (5)
I believe the answer is:
facet
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">side of a cut gem</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 facet that I've seen before include "side of rock" , "(Gem) side; aspect" , "Angle" , "Side or surface, diamond or building" , "Flat surface on a cut gem" .)