Before you empty the trash, it’s a good idea to double-check what you are deleting. You can view the contents of the trash bin by clicking File > Show Trash…, by using the keyboard shortcut Apple + Shift + T, or by clicking directly on the trash can.
This will open a separate window, which displays all of the video clips in the trash and even allows you to watch them before deleting them for good.
You can empty the entire trash bin by clicking Empty Trash… (keyboard shortcut: Shift + Apple + Delete), or just delete certain clips by selecting them and clicking Delete Selected Clip…
