Side of a cut gem (5)
I believe the answer is:
facet
'side of a cut gem' is the definition.
(I've seen this before)
This is all the clue.
(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" .)