First you need to determine what the brown spot consists of. If it's starch, it should just wash out with soap and water. If it's fusible, you can try a couple of different things, keeping in mind that the piece is pretty much toast anyway, so you might as well try whatever. If you have some acetone nail polish remover, dab some on with a q-tip. I've had success with it. Just make sure to keep the acetone away from any heat source (i.e. your iron) until it evaporates.