You mean like this ?

First, whichever clock app you are using, check for "Always on Top" in it's preferences(hopefully it should be there), and enable it.
PS: Doesn't work for Win Gadgets.
Otherwise, download
Rainmeter, an open-source desktop styling app. It'll take some time for you to get familiarized with its functioning. After installing and running the app for the first time, just right click on the clock widget > Settings > Position > Stay Topmost.