Matelot with no mates finding mollusc (7)
I believe the answer is:
abalone
'mollusc' is the definition.
(abalone is a kind of mollusc)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for abalone that I've seen before include "Mollusc with an oval shell lined with bluish mother-of-pearl" , "Ormer, ear shell" , "Ear-shaped shell" , "something to eat from the sea" , "Edible marine mollusc -- Oban ale (anag)" .)