Chowder ingredients (5)
I believe the answer is:
'chowder ingredients' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for clams that I've seen before include "Edible bivalve molluscs" , "Marine bivalves" , "Their shells are firmly closed" , "Proverbially retentive molluscs" .)