Shellfish left in river (4)
I believe the answer is:
clam
'shellfish' is the definition.
(clam is a kind of shellfish)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for clam that I've seen before include "A silent type" , "Proverbially reticent marine mollusc" , "One is as uncommunicative as a mollusc" , "Mollusc with tightly-closed shell" , "Bi-valve with firmly-closed shell" .)