Hi there,
I also sent a query to the clip vendor, and even though the description had indicated “alpha channel” included, it turned out that the clip I purchased did not have it! So that explains a lot! Once I understood that it wasn’t something I was doing wrong, I went back and found a more appropriate tutorial (remove black background) using Luma Keying and WAS successful (after a little fiddling and a few false starts) so that my clip is now on a white background! So we’re all good! Thanks.
Baruch