Bill James came up with a new definition of clutch a few years back, and when applied to MLB hitters, he found that there truely are players that are clutch.
He claims we have the whole concept of clutch wrong. He claimed that clutch has to do with two things, the leverage if the situation in the game, and the importance of the game in the standings.
Basically, he argued that the goal isn't to score runs, the goal is to make the playoffs, and the way to do that is to win games. So, a homer run with the bases loaded of a 7-0 Royals/Astros game in July isn't as clutch as a walk to lead off a tie game between the Reds and Pirates in September.
He weighted every plate appearance based on how clutch it was according to his new definition, and compared how papers did in the highly clutch situations vs regular situations. He found that there were hitters, like David Ortiz, who did much better in James' clutch situations than they did in non-clutch situations. And there were players, like A-Rod, who did much worse in his clutch situations than they did in non-clutch situations. If you saw the list, it came pretty close to matching what people intuitively thought were the clutch hitters.
I think the most intersting part of his new definition of clutch is that he didn't tie it to run production, but instead to importance of the situation. So anyone who did well in those high pressure situations were clutch, even if they didn't drive in any runs.