Merriam-Webster's Advanced Learner's Dictionary:
1clam /ˈklæm/ noun, pl clams
1 [count] : a type of shellfish that lives in sand or mud, has a light-colored shell with two parts, and is eaten both cooked and raw
• steamed clams
• clam chowder
- see color picture
2 clams [plural] US slang somewhat old-fashioned : dollars
• He won 20 clams [=(more commonly) bucks] playing poker.
(as) happy as a clam US informal : very happy
• She spent the afternoon reading and was as happy as a clam.