Light-splitting glass (5)
I believe the answer is:
prism
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">light-splitting glass</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 prism that I've seen before include "Cut-glass object" , "Transparent block" , "Theatrical Miss" , "Light-refracting element" , "Glass triangle that separates white light into the colour spectrum" .)