This is difficult, because X11 expects that the way you do this is to unmanage it *and* that a window that is unmanaged knows how to manage itself. In particular, the X server may well send focus to a managed window *itself* via its window focus inheritance policy. The best xmonad could do would be to use a handleEventHook to detect the window being given focus (this might be rather difficult because focusIn and focusOut events are not currently delivered by the Haskell X11 bindings, sigh) and send focus elsewhere --- but in that case the window would still briefly receive focus.

You probably need to find a clock program that knows how to operate as an unmanaged window.