Shell material (5)
I believe the answer is:
nacre
'shell material' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nacre that I've seen before include "valuable material" , "Mother-of-pearl from shellfish" , "Mother-of-pearl - crane (anag)" , "inside of shell" , "Constituent of inlay furniture" .)