Imitation diamond (10)
I believe the answer is:
rhinestone
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">imitation diamond</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 rhinestone that I've seen before include "Rock" , "it's made of paste" , "imitation gem" , "Imitation diamond" , "one not really brilliant?" .)