You have two options to consider. If you only want the clock on your dock then Live Dock Clock for 0.99c is the app for you. If you want a tonne of additional features then possibly Dock Clock the 1.99 c version is a better choice. Look at both apps carefully and make up your mind which app fits your needs more appropriately. We had so many users asking that Dock Clock can't be used only on the Dock and that 0.99 c would be a more appropriate price for just the clock on the dock. So we listened and built Live Dock Clock for 0.99 c.
Features include
+ 11 clock designs
+ Easy to setup in Preferences
+ 12 / 24 hour settings
+ Date and time displayed in Dock

Requirements: Mac OS X 10.6 or later