I don't know how to make a mouse click simulate a backspace key press in any situation. But you probably want this for one certain application, and there is probably another way to do what you need.
What do you want the backspace for? Do you want it to navigate back in the browser history? Or to delete text in a textbox/area?
If its to navigate backwards, its possible through other means besides simulating a backspace key press. If its to delete text you run into a little bit of a problem because a mouse click is also used to position the cursor in text areas, focus elements etc. You could put an onClick event handler on the specific textbox/area and just manipulate the value attribute of the element to contain one less character.
If you have questions about how any of this is specifically done, you can post again.
cheyney