Select Archive ClipHistory in the Collection menu to save the contents of the ClipHistory into an AceText collection file and then start with a blank ClipHistory. You will be asked for the name and location of the file that you want to archive the ClipHistory into. If you tick the "open the saved file" checkbox in the file selection dialog, then the archived ClipHistory will be opened into a collection tab.
There is no toolbar button for the Archive ClipHistory command. If you've hidden the main menu, you can access the command by right-clicking on the ClipHistory tab.