One of Korea's must eat dishes is Chogaejjim and a good place to get it is at Golmokgil over by Nonhyun Station. You get a huge bucket of various steamed clams, giant mussels, scallops and more. The broth is later used to make a satisfying kimchi noodle soup.
It's not cheap (starts at 49,000) but it is a unique and delicious way to eat clams. It is best in winter so go now.
Address: South Korea Seoul Gangnamgu Nonhyeondong Eonjuro147gil 9-3