Is actually possible and quite easy.
You hold your thumb against your middle finger, possibly supporting them with the ring finger. You keep your index finger relaxed and throw your hand down fast and stop the motion suddenly, which causes the index finger to slap against the middle finger.
Happy clapping.
Thanks for posting, kke, and welcome to the Straight Dope Message Boards.
Here’s the link to the column you’re presumably commenting on: What is the sound of one hand clapping?
I have moved to the appropriate forum, since this is a comment on a column by Cecil, not on a Staff Report.
And, BTW, what you’ve described is not inherently different from snapping your fingers, which aint the same as “clapping.” It’s not about “what kind of noise can you make with one hand” – it’s about the clap you make by bringing two hands together.
The true answer, which I have received after much introspection, is to use two people. Sure, that’s two hands, but you’re only using one. Consider THAT mystery solved. 