Complex sentence: Supernumerary is a non-regular member of a staff, or member of the staff or an employee who works in a public office who is not part of the manpower complement: a temporary employee in addition to the permanent staff.
Split into simpler sentences: Supernumerary is a non-regular member of a staff, or member of the staff or an employee who works in a public office who is not part of the manpower complement. A supernumerary could be an extra member or a temporary employee in addition to the permanent staff.