I copied my password to the clipboard! Can you delete it?

PowerShell, 3 bytes

scb

That's a default alias for Set-Clipboard. When called with nothing piped in and no arguments supplied, it blows away the current clipboard contents.

Vim, 3 bytes

"*Y

Sets the clipboard content to a newline.

By default, vim opens an empty buffer on start up. Now the program Yanks (copy) the entire line (including a newline) and store it in your clipboard by yanking it the text into register "*. The * can be changed to a + for the other clipboard.

Javascript (Chrome DevTools Console), 6 bytes

copy``

Note that this is a feature of the console (which also works in the FF console) and is not part of the language standard.