Well, let me be the dissenter, because I *DO* believe in "clutch." I believe that some players rise to a moment - that while their contributions are steadily good, in the proper moment, at the time when the game is to be decided, they make the momentum play - the play that shifts the game. Those momentum plays are, to me, what define clutch.
Too many folks look at clutch as the "last x seconds of a close game." To me, that constitutes an invalid look at clutch. You're looking at a performance across a time, rather than the making of the "one or two big plays that change the game."
Those of you who've played know what I'm talking about; you've been in that situation where your team is down, you're starting to fight back, and then, all of a sudden, there's "that guy" - he catches the ball, and you think to yourself as he's shooting, "Oh no. Not like that. We can't give *THAT* up." You feel your heart sink as the shot goes up, and when it comes down - and drops in - you're completely deflated. And you knew what was happening the minute he touched the ball. And you knew it was going in from the moment he released it. Maybe it's early in the game, but it's the play that just completely takes the wind out of your sails. And over and over and over again, when you deal with that particular player, you find him making that critical play - over and over and over and over again.
And here's why that's NEVER going to show up in the studies: 1) it happens at different times throughout the game, 2) studies measure performance at defined times, and compare that against times that "aren't" that, but they fail to look and see the PERCENTAGE of times the player in question is involved in the plays across the two, 3) there is a huge mental component to this, not only on the part of the player in question, but on the part of his opponents, and 4) even BROKEN plays can sap the strength of an opponent (i.e. even though the player missed the shot, the very fact that he got it triggers the belief - which becomes self-fulfilling prophecy - with the opponent that they've lost the game).
If I had to define clutch, I'd define it this way: the propensity of a play by the particular player to reshape belief of team and opponent that the eventual outcome is now decided (in favor of the player's team) when previously it had not been. In other words, the ability to deflate the opponent at the game-deciding juncture.