I'm not sure from whom this particular saying originates but it is normally attributed as a Zen Buddhist Koan, which are meditative tools designed to shock the enlightenable mind into awareness. These may have no answer, which does not mean they are inanswerable. It simply means that there may either be no right answer or any answer that reflects that the mind has arrived on par with the question and its design is right. I would not think of them as nonsensical or rhetorical or even really questions/statements but as meditative tools designed to take the meditative mind to a specific place.

It is nonsense. Just look up the word "clapping" in a dictionary to see the problem. One might as well say, Colorless green ideas sleep furiously. Or read some Lewis Carroll.

I rapidly and strongly pull all the fingers except for my thumb against the palm of my hand, and slap my palm with them. It makes an audible sound if you do it strongly enough. And that is the sound of one hand clapping. Where there's a will, there's a way.

My reflexive reaction when I hear that one is to slap the speaker in the face.

I see you do not appreciate imagination.

On the contrary, melonkali is trying to demonstrate the sound of one hand clapping for the person who is asking the question. It seems entirely appropriate to me.