Emptying trash can of an MP4 file

I have a mysterious MP4 file that I cannot get rid of, apparently because it is constantly and persistently in use, though what it is doing eludes me. The name of the file is p02cyg5.mp4, but I suspect that means nothing. Double clicking on the file starts a long conversion process producing a white window with a time bar and a play/pause button Starting the file produces no picture and no sound. The time bar indicates a total of 57 minutes. (I would upload an image of the file's info, but the Apple equivalent of Windows' quite easy to use PrntScrn function seems to be problematic.)