Bivalve left in river (4)
I believe the answer is:
clam
'bivalve' is the definition.
(clam is a kind of bivalve)
'left in river' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'river' becomes 'cam' (I've seen this before**).
'l' placed within 'cam' is 'CLAM'.
(Other definitions for clam that I've seen before include "Proverbially reticent marine mollusc" , "Large shellfish with hinged shell" , "Shellfish - secretive person" , "Tightly closed shellfish" , "Mollusc, one proverbially reticent" .)