Marine bivalve with a fluted fan-shaped tail (7)
I believe the answer is:
scallop
(Other definitions for scallop that I've seen before include "Shellfish that's also plc" , "Shell - sign of a pilgrim" , "Bivalve with radiating ridges on shell" , "Mollusc or lacy edge" , "Bivalve mollusc with fan-shaped shell" .)