This year, Daniel Murphy of the Mets is the poster child for the clutch hitter. He practically was the entire Mets offense in the clinching game 5 of the NLDS against the Dodgers. He's hit home runs in five straight games, including against all three of the NL Cy Young candidates. This from a man who has never hit more than 14 home runs in an entire season. (I enjoyed seeing it against the Dodgers, but can't say I'm happy that he's still doing it to my Chicago Cubs in the NLCS.)
That kind of long-term view doesn't tell you anything about how a player or team will perform in a small group of games - such as a playoff series. Those three rounds of playoff series (four if you count the one-game wild card) every year are in large part a measure of which players and teams are on one of those stretches of great play. As it's been said of all kinds of sports: "It's not who you play, it's when you play them."
People have tried to measure this "clutch ability" in all kinds of different ways. (Of course they have; everything is measured in baseball.) There really hasn't been any success in this area; every time some analysis looks to be doing a good job, something unexpected happens to throw it off. And that's to be expected. The game of baseball is largely predictable in the long run; and largely impossible to predict over the short term.
So call it "intangibles" or "clutch" or plain old luck. Sometimes one player, or one team, will just be great over a short period of time. All we fans can do is hope that our guys are the ones with the right stuff at the right time.