The term 'Stuart Portrait Subject' refers to a person who was the subject of a portrait painted by an artist in the Stuart period. The crossword solution for this term is 'REVERE', which is a six-letter word. Paul REVERE was an American silversmith, engraver, early industrialist, and Patriot in the American Revolution. He is best known for his midnight ride to alert the colonial militia to the approach of British forces before the battles of Lexington and Concord. He was also a portrait subject of the Stuart period, making him the answer to the crossword clue.