XFCE version: 4.8.3 on Ubuntu 12.04.
1. Open XFCE -> Settings -> Settings Manager.
2. Click on window manager
3. Switch to keyboard tab
Buttons Clear and Close has the same keyboard shortcut: Alt+C.
Clicking on Alt+C closes the dialog, there is no ability to invoke clear using keyboard.
The bug still exists in 4.10.
Thanks for your bug report. I'm marking this as a duplicate ; the fix is not trivial as those buttons are both stock GTK+ buttons.
*** This bug has been marked as a duplicate of bug 8232 ***