As Mike suggests, PS version is key if you want to use Set-Clipboard as it was introduced in a later version (5.0?). Although I would suggest using $null rather than an empty string or one with a space in it.
Set-Clipboard $null
If the cmdlet isn’t available, you should be able to pipe to clip.exe in PowerShell without any problem if the cmdlet is not available to you. The example you gave isn’t working because in PowerShell echo is an alias for Write-Output.