If you want to clear the translation box, it is highly recommended not to do this manually, but via the menu command MetaTexis | Copy and delete | Delete translation (default shortcut: ALT+Shift+Delete, button on the MetaTexis toolbar).
This is especially important in cases where you want to get rid of segments which were retrieved from a translation memory. Only via the command MetaTexis | Copy and delete | Delete translation can you make sure that the translation box is cleared properly.