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 will be permanently deleted.