Just for clarification: When you issue the slackpkg clean-system command, unselect any third party package(s) which you do NOT wish to be removed, before you continue. If you do not, this command will remove them. By default, they are all selected for removal.
Careful with the clean-system command!!!! The purpose of slackpkg clean-system is to remove all third party packages from your system. If you do a lot of experimentation you can use it to revert to a stock slackware installation. If you are trying to remove packages you no longer want, you will be better off with 'slackpkg remove <package name>'
'slackpkg help' will provide information on the various commands within slackpkg. I would avoid using clean-system unless your intention is to remove all thirdparty packages, because that's why it is there