The nice thing about digital scrapping is that there are several ways to get the same end result. I usually place the paper clip where I want it on my picture [on its own layer] and then just erase off the part that would fall to the back of the picture. A suggestion that I saw recently was to set your eraser to 60 or 70%, which would leave just a shadow that would look more realistic. I haven't tried that yet.
Here is a tutorial by Tonya Doughty which is yet another method.
http://www.ssreflections.com/tonya_aug04.html
HTH