Last chapter went to @AnotherClaraOswald. But, because I'm a die-hard Doctor/River fan (and because of the episodes), I don't think that he necessarily likes her back that way. She blurted out that she fancied him in the Christmas Special under the truth field, although I have a feeling that'll fly out the window with Capaldi. Now, for the question.

Who/what was the first thing the Doctor had to let die in season 5?

Again, first person to answer correctly gets the dedication!