Clam Session is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

As this creature enters, choose a word. At the beginning of your upkeep, you may sing at least six words of a song, at least one of which is the chosen word. If you don't, sacrifice this creature. You can't repeat a song.