Mollusc left in river (4)
I believe the answer is:
clam
'mollusc' is the definition.
(clam is a kind of mollusc)
'left in river' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'river' becomes 'cam' (river in Cambridge, England).
'l' going into 'cam' is 'CLAM'.
(Other definitions for clam that I've seen before include "Proverbially reticent marine mollusc" , "Mollusc with firmly-closed shell" , "Tightly closed shellfish" , "Quahog" , "Shellfish - secretive type" .)