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">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for nacre that I've seen before include "inside of shell" , "Constituent of inlay furniture" , "valuable material" , "Pearl's mum" , "Mother of pearl" .)