Meg: You remember me? Because I sure remember you, Clarence.
– , 6.10 Caged Heat
"Clarence" is a nickname for Castiel used by the demon Meg. It was first used in the episode 5.10 Abandon All Hope..., and then in the episodes 6.10 Caged Heat, 7.17 The Born-Again Identity, and 8.17 Goodbye Stranger.
The name is a a reference to the movie It's A Wonderful Life, in which the angel Clarence shows Jimmy Stewart that life is worth living, and then finally gets his wings and goes to heaven.
In 9.03 I'm No Angel, Castiel adopts the name Clarence as a cover when he is human.
See also : Nicknames