Clipping is simply the amplifier trying to output more power than it's power supply is capable of. It is not a function of slew rate but simply the amplifier is being asked to provide more power than it's power supply is able to handle. The term clipping came into use because of the oscillographic picture of a sine wave output of an amplifier in clipping looks as if it had the top cut off. Clipping causes distortion based in the odd harmonics. Slew induced distortion or Transient Intermodulation Distortion is a function of amplifier bandwidth or speed. If an amplifier has power in reserve it cannot be considered in clipping until that reserve is used up. That is why the spec of headroom is important. If an amplifier has a headroom spec of 3db it can pass 2 times the power it is rated for. Unfortunated speaker loads are not linear over frequency. While the nominal impedance may be 8 ohms the impedance may at some frequencies be as high as 20 ohms or as low as 2 ohms. It is possible that at the majority of frequencies our amplifier can supply enough power to drive the speakers but if the impedance requires more power than the amp can muster, the result is, you guessed it! Clipping.
