Playing marbles (6)
I believe the answer is:
agates
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">playing marbles</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for agates that I've seen before include "Banded semiprecious stones" , "Stones to be found in a stage" , "Multi-coloured quartzes" , "Name for stones" , "[RING]-fillers required" .)