You can try a filter like the Notch effect or NewBlue Hum Remover in Premiere Elements.
They're not easy to use. You basically have to figure out what frequency the hum is at and you set the filter to remove all audio at that frequency without affecting the rest of the audio. There is no way to do this automatically. And, if you set the filter to take out too wide a range of frequencies, it can make the rest of your audio sound a bit strange.
But removing a hum is exactly what these two audio effects are designed to do.
