On X11 you can do better: the clipboard isn't actually stored anywhere, the program requesting the paste gets the data directly from the program that claimed the clipboard.

The clipboard lib you're using just spawns xclip (or xsel) on Unix. xclip has a -loops argument that is the maximum number of pastes. So you could just spawn:

  xclip -loops 1 -in -selection clipboard

And the password would be available to paste exactly once. Unfortunately I don't think xsel has any such option.

There are clipboard manager apps that could grab the paste immediately and persist it, but I'm not sure how common they are or if they are default anywhere.