Clamshells meaning : The shell of a clam. A dredging bucket with hinges like the shell of a clam. In food service, the closing box (usually styrofoam but sometimes cardboard) given to consumers with take-out food. Any object that has a hinge on one edge and two surfaces that close together. An amphitheater, especially an outdoor amphitheater; the semi-circular acoustic backdrop behind and above the performers.