Working clandestinely is working in Belgium as a non-Belgian without the permission to work in Belgium. If you are neither a Belgian nor a citizen of a member state of the European Union, you must obtain permission to work from the government. Some foreigners do not need such permission. All the others – and especially if you come from a country outside the European Union – must get permission. Those without a residence permit cannot get this permission. If you work without permission you are working ‘illegally’. Your boss can be punished for this. Clandestine work is almost never declared to the authorities by the boss. If you do work without legal residence, you are therefore probably also working off the books.