First you need to have the side bar showing. If its not showing, go into Control Panel/Windows Sidebar and check the Start with Windows box. Now make any change (say select a different position for it) and click apply- this will make it visible. Change the position to where you want it, and click OK.
The clock is on the sidebar by default, but if its not there, you can put it there by right-clicking the sidebar and choosing Add Gadgets.
BTW if you calculate the percentage of screen taken up by the clock, and then calculate how much that cost you, I think you'll find it's much cheaper to buy a wall clock (I got one for 1 Euro in IKEA)
:-)