Layer of molluscs damaged porthole frame (6-2-5)
I believe the answer is:
mother-of-pearl
'layer of molluscs' is the definition.
(layer in the shell of some molluscs)
'damaged porthole frame' is the wordplay.
'damaged' indicates an anagram.
'porthole'+'frame'='portholeframe'
'portholeframe' is an anagram of 'MOTHER-OF-PEARL'.
(Other definitions for mother-of-pearl that I've seen before include "Internal layer of shell" , "Decorative substance got from certain molluscs" , "Nacre" , "Iridescent material used in ornamentation" , "girl's mum" .)