It's a terminal command,
/System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine -background &
The animations all work smoothly. The screensaver you get will be the one you have set in System Prefs > Desktop and Screensaver, so make sure that's set to Screensleeves. The theme and settings will be as per your options there (including whether it shows a clock or not).
Protip: Make sure your music player of choice is already running and playing when you use that Terminal command. If you want to stop the 'desktop screensaver' you'll need to use Activity Monitor to find the 'ScreenSaver Engine' process and kill it.