There seem to be a number of gear combination sets available. For example, check out
http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=4950
You don't have to have exactly a 2:1 ratio on the gear teeth. If you match 32 teeth to 8 teeth, you would move your servo through 90 degrees to get a 360 rotation on the clock hand. Do the math if you get gears with other ratios.
[edit]Whether or not you can move the servo 90 degrees is another question, as Mike Mc has pointed out.[/edit]