When you delete a clip, it is not really deleted but merely moved into the recycle bin. You can restore the clip from the recycle bin, until it is purged from the recycle bin. You can configure AceText to purge old clips from the recycle bin in the Files Preferences.
To restore a clip from the recycle bin, use the 
Move Clips button to move it back into a collection. You can restore the clip into any collection, not just the one you deleted it from.
If you click the 
Delete Clips button in the recycle bin, the selected clips are permanently deleted once you close AceText. While AceText is still running, the deleted clips may linger in the undo history. The recycle bin has its own undo history and you can undo deleting clips from the recycle bin like you can undo deleting clips from a collection.
The undo history for each open collection in AceText keeps it own copies of all the clips you edited or deleted. The 
Undo and 
Redo commands for the ClipHistory and your collections do not depend on the clips being in the recycle bin. But restoring a clip with the Undo command requires you to first undo all later changes you made to the clip's collection. Restoring a clip from the recycle bin doesn't require you to undo anything. A collection’s undo history is discarded when you close the collection. The recycle bin can be configured to persist.
The toolbar on the Recycle Bin tab page shows the following buttons:
Delete Duplicate Clips
Cut Clip to Clipboard
Copy Clip to Clipboard