It's a reference to an old movie titled "It's a Wonderful Life," in which there is an angel named Clarence that comes down from Heaven and helps out a guy named George realize his worth and how many lives he has changed and affected just by being born.
Have you ever seen the movie "Its A Wonderful Life" At one point Cas basically says he doesn't understand what she is referencing and she says that he should really watch a movie.In the movie, there is an Angel names Clarence.
Clarence is an angel from "It's A Wonderful Life". He is a guardian angel to Gorge Bailey who has given up his dreams in order to help others, like Dean and Castiel's relationship. That's my theory, Hope it helped!