Mother of pearl (5)
I believe the answer is:
nacre
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mother of pearl</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 nacre that I've seen before include "inside of shell" , "Pearl's mum" , "valuable material" , "Mother-of-pearl from shellfish" , "Mother-of-pearl - crane (anag)" .)