For Gboard, tap the 3 dot menu button just above the keyboard (to the left of the mic icon), then tap Clipboard. You'll see all of the saved items there. Long-press each one and tap Delete. I don't think there's a way to mass-delete clipped items.
ncG1vNJzZmien6fCrr%2BNmqWdqp%2BesaSxza2pmqRemLyue9OhqZ6ZlKh8pLjIqWStqpGue3J8k3Bva3Bf