The Clover Club was a men's club that met at Philadelphia's Bellevue-Stratford hotel from the 1880s through the 1920s. The group consisted of journalists, lawyers, socialites, and other important men about town -- obviously they needed their own signature cocktail. And it was a good one, regarded as one of the gems of Pre-Prohibition America.
Clover Club Cocktail
2 oz Gin
3/4 oz Lemon Juice
3/4 oz Raspberry Syrup
1 Egg White
Dry shake (i.e., shake without ice) the ingredients for a good ten seconds. This will cause the egg white to froth, giving the drink a nice foamy texture. Add ice and shake again. Strain into a chilled cocktail glass.
There's always a small risk of bacterial contamination when using raw eggs in a drink, but it's a very small one. (And mixing the egg with alcohol reduces the risk even further.) If you're concerned, you can always use pasteurized egg whites instead.
If you don't have raspberry syrup, it's become common to subsitute grenadine, although I don't think the resulting cocktail tastes quite as good. In The PDT Cocktail Book, Jim Meehan recommends using 1/2 ounce simple syrup and one barspoon raspberry preserves.