[BMW]

I'll add some interesting data to it, just because research is fun.

I searched Newspapers.com and the earliest reference that comes up to the term "clutch hitter" is 1930. It was referring to Allen Elliot, the first baseman of the Waterloo Hawks in a contest against the Cedar Rapids Bunnies (yes, really the Bunnies) of the Class D Mississippi Valley League. It makes no mention as to what "clutch hitter" means.

In the mid to late 1930s, you see it used a bit. In all the articles, they never define it specifically beyond pure results. Either they:
1. Mention a guy is a clutch hitter.
2. Mention a guy is a clutch hitter with the example that he gets a lot of RBIs, especially from situations with multiple runners on base.
3. Mention a guy is a clutch hitter with the example that he gets RBIs that put his club ahead or "when they count the most".

There's no instance that I found where an article explicitly referred to any intangible trait the player had, although it's hard to tell if the authors were assuming that the reader knew that "clutch hitter" meant some kind of intangible ability.

An example: "Goslin became a major factor in the Detroit pennant drives of 1934 and 1935, living up fully to his reputation as a great clutch hitter. In the World Series of 1935 he drove the run which won the World Series for Detroit and became the toast of the city's ardent fandom."

"Sullivan is a Clutch Hitter: The only time Billy Sullivan, St Louis Browns catcher, delivered as a pinch hitter in 1938, he hit a home run." (Oddly enough, he was 3 for 10 with a walk and a home run as a pinch hitter in 1938. It was a solo shot in the 9th inning that tied the game 7-7 which the Browns would lose in the bottom of the 9th, 8-7).