WD40 won't do anything but make the belt slip. It's primary purpose is a lubricant/cleaner, don't forget. If the belt is that loose, get a new one. If the new one is loose, you'll have to do a custom resize, meaning going down a couple numbers lower on the belt size. Every letter and number in a belt part number means something, you can choose smaller sizes to fit the application better. The parts store guys could explain it better than I can.