Source : Webster's Revised Unabridged Dictionary (1913)

Clambake \Clam"bake\, n.
   The backing or steaming of clams on heated stones, between
   layers of seaweed; hence, a picnic party, gathered on such an
   occasion.

Source : WordNet®

clambake
     n : a cookout at the seashore where clams and fish and other
         foods are cooked--usually on heated stones covered with
         seaweed