How to define an onMouseDown area?

I’m creating an animated menu that I’d like to ‘stow away’ when the user clicks anywhere but the Menu button.

I know this involves adding a mouse listener for an onMouseDown event, but I’m not sure what or how to define this…

Is it possible to define an entire area (my site) minus a specific area (the menu button) for an onMouseDown event?

If someone would please help me with this script I would really appreciate it!