Edible shellfish, source of mother-of-pearl (7)
I believe the answer is:
abalone
This clue is a double definition.
'edible shellfish' is the first definition.
'source of mother-of-pearl' is the second definition.
(I've seen this before)
(Other definitions for abalone that I've seen before include "Would you be able on a US shellfish?" , "paua" , "shellfish for instance" , "gastropod" , "something to eat from the sea" .)