am, clipping is refering to the signal being clipped. the wave is like.. /\/\/\/\/\/\/ well sorta. as u raise the volume ( power) to a speaker the wave gets larger. when an amp only pushes 600w but its trying to push say 900w the wave gets too large for that amplifier to produce. so the top and bottom of the wave are cut off or "clipped" if u are severely cliping a signal some refer to you as sending a square wave to the sub. what they are saying is the signal is being so clipped it looks almost like a square. for example. take a triangle. now cut a large portion of the top and bottom off. now it looks similar to a square. that signal is not clean and thats why u fry ur amps and subs when you do this.
about how much a sub has to be underpowered... do u mean in order for the clipping to not hurt the sub? well bcae1 says if the amp is 1/4 of the subs rms power then even at full clipping the sub wont get hurt. but it will sound like h3ll so why would you want to do that anyways? its better to get a more powerful amp and have headroom than too weak of an amp....