Oyster's gem (5)
I believe the answer is:
pearl
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">oyster's 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 pearl that I've seen before include "Gem; an attacked Harbor" , "piece of wisdom" , "Oyster growth" , "Small type" , "Much loved and valued person" .)