The Paste Text into Clip item is available via the Insert and Extract Text button on the collection toolbar, except when the Recycle Bin tab is active.
Select the Paste Text into Clip item insert the text on the clipboard into the text of the active clip. You can also paste text into a clip by pressing Ctrl+V on the keyboard while editing the clip's text. If you want to create a new clip, use the Paste as New Clip button instead.
See Editing Clips and Text to learn how to use AceText.