Got it!

    def action_copy(self):
        entry = self._selected_entry()
        if entry is None:
            return
        if pyperclip is None:
            self.notify("pyperclip not installed", severity="error"); return
        pw = entry.password or ""
        try:
            pyperclip.copy(pw)
            self.notify(f"Password copied - clears in {self.CLIPBOARD_TIMEOUT}s",
                        severity="information")
            self.set_timer(self.CLIPBOARD_TIMEOUT, lambda: self._clear_clipboard(pw))
        except Exception as exc:
            self.notify(f"Copy failed: {exc}", severity="error")

    def _clear_clipboard(self, expected):
        if pyperclip is None:
            return
        try:
            if pyperclip.paste() == expected:
                pyperclip.copy("")
                self.notify("Clipboard cleared", severity="information")
        except Exception:
            pass

Just to make sure..

 [plan@toolbx kdbxview]$ ls -lh ~/Passwords.kdbx && file ~/Passwords.kdbx
-rw-------. 1 plan plan 13K Jun 12 12:15 /var/home/plan/Passwords.kdbx
/var/home/plan/Passwords.kdbx: Keepass password database 2.x KDBX
(.venv) ⬢ [plan@toolbx kdbxview]$ ss -tunp 2>/dev/null | grep -i python
(.venv) ⬢ [plan@toolbx kdbxview]$