The term 'Muslim Sovereign' refers to a ruler of a Muslim country, and the crossword solution for this term is 'Sultan', which is a 6-letter word. A Sultan is a title given to a ruler of a Muslim country, and is derived from the Arabic word for 'AUTHORITY' or 'POWER'. Historically, Sultans have been the rulers of many Islamic countries, including Turkey, Morocco, and Oman. In modern times, the title is still used in some countries, such as Brunei, Malaysia, and Saudi Arabia.