While I am a strong proponent of clicker training, and do clicker train a stack for my showdogs, I wouldn't recommend the clicker right out of the gate. You should really teach the stack by itself, and use the clicker to refine it. Many boxers have very short attention spans, and it helps to have a frame of reference to work from when you are training something like a stack.
Stacking isn't really rocket science. It's just a matter of placing the feet where they go, get the head in nice position (high and chin down) and teasing the tail up. Top that off with lots of praise and treats, and you got yourself a show stack.
good luck.