Some mollusks (5)
I believe the answer is:
clams
'some mollusks' is the definition.
(clam is a kind of mollusk)
This is the entire clue.
(Other definitions for clams that I've seen before include "Marine bivalves" , "Proverbially retentive molluscs" , "Edible bivalve molluscs" , "Their shells are firmly closed" .)