Thin plate (6)
I believe the answer is:
lamina
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">thin plate</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for lamina that I've seen before include "Flat plate" , "Thin plate or layer" , "Thin plate or scale" .)