Clint is a vampire, which is how/why he's still amazingly alive with all the stunts he pulls (throwing himself off buildings to make the shot, etc.). The Avengers all know and really it just means he fits in that much better with the rest of the them and their powers/longevity. They take his immortality for granted, and kind of forget that vampires still have that one, great, weakness (What? I have no idea - go wild!) Baddie somehow knows about Clint and targets his weakness and the others are all stunned when he goes down...