As the title says, I'm looking for a way to clip a Sky Texture Background to above the Horizon only, anything below it should be black. I tried using the clip function of the mapping node but the result is something else. I also tried combining the sky texture with an rgb node with a mix shader and using the mapping node as Fac but didnt find a solution. Please show me a working node setup.
You can use Generated texture coordinate with World to read Z axis on -1 to 1 range. Here -1 is straight under, 1 is straight up and 0 is the horizon. Taking it from there, you could simply use Math node to test for Greater than 0. Math node returns 0 for under 0 and 1 immeadiately above 0 so you can achieve a sharp mask.
For smoother options you could use Color Ramp node, it even conveniently ignores anything below 0 in the range.
EDIT: Here are couple setups for Color Ramp. First ColorRamp range from horizon to Zenith, horizon is on the left of the ColorRamp at value 0.0.
And here we have shifted the -1 to 1 range between 0 and 1 by adding 1 and multiplying by 0.5. Horizon is in the middle of the ColorRamp at value 0.5.