Use Geektool to get the time, then you'd have to Photoshop the front layer that you want the clock to go behind 'out' of the picture and put it on a new layer and save as a separate image.
Then use that image in Geektool and position it on top of the original wallpaper with the clock behind.
i am using leoparddocks application which easily allows you to put spacers. there is also a way using the terminal but i dont remember what you had to type.