Mother of pearl (5)
I believe the answer is:
nacre
'mother of pearl' is the definition.
(I've seen this in another clue)
This is all the clue.
(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)" .)