Digital Light & Color

1. start the Clone Tool,
2. load a Source image other than the Output and the Input images -> the tool generates a small source image thumbnail
3. repeat the step 2. a two or more times -> now you have three or more small source image thumbnails
4. Click OK
5. delete one of the big thumbnails used as a source image -> the clone output is deleted

Deleting an image loaded as a source image, even if the image itself was not finally used for cloning, deletes the clone output with all the dependant thumbnails and masks.

One more comment on it:
It would not be a problem if you use one or two images as Source Images. But if you had more versions of a source image trying to find which one suits the best for cloning, you end up with a train of small source image thumbnails and a number of source image versions, out of which only one will finally be truly dependent. The rest will become redundant and disposable, but remove them from the browser would mean lose also the result.

By the way, the step 4. is not necessary.