The Far Z Clip Plane in the Hammer Editor is used to define the maximum distance from the camera that objects in your level will be rendered. This can help to improve performance by not rendering objects that are too far away to be seen by the player. Here's how to use it:

Open your map in the Hammer Editor.

Select the "Tools" menu and choose "Options".

In the "General" tab, look for the "Far Z Clip Plane" option.

Set the value to the maximum distance from the camera that you want objects to be rendered. The default value is 32768 units.

Click "OK" to save the changes.

Save and compile your map.

Note that setting the Far Z Clip Plane too low may result in objects disappearing before they are out of sight, which can look strange and affect gameplay. Setting it too high can impact performance, so you should find a balance that works best for your map. Also, keep in mind that the Far Z Clip Plane affects all objects in your level, so you should adjust it based on the size and complexity of your map.