If you are trying to clear the clipboard inside of a script, you likely need to do something like the article below shows since it requires it be essentially off a Windows Form thread.

https://stackoverflow.com/questions/3931729/how-to-clear-the-clipboard-contents-using-c-sharp

If you want to do it in an automation, the easiest way is to use the Click event from a button on a form to trigger the method. In your automation where you wish to clear the clipboard, you would call PerformClick() on the button. The click event of that button would then fire and clear the clipboard. The trick here is that the button must be "Visible"; meaning that it must be on a form that is not hidden and the visible property of the button must be true. If you don't want a user to be able to see the form, you could set its Opacity to 100% and it would be functionally invisible. You can also place the button on a Form behind another UI element and set its TabStop to false so that it is technically visible, but impossible for a user to click.