1: When a variable is replaced by one number, the highest figure becomes the lowest (7)
2: An Asian making money carrying a letter from Greece (7)
4: Book's conclusion darker, having more twists (7)
5: Seal simpers forlornly (7)
6: He explored star (7,6)
7: Plain gold and sterling - the ultimate in elegance (7)
8: Beginning to send poor around north and south for support (7)
10: Changed musician not so offensively pious (13)
16: Ignore the first of itineraries for excursions (6)
17: Bad blood from taking head off to slay William (3,4)
18: One American tree growing wild that's very plain (7)
19: Maiden from France lowers bloomers (7)
20: Loss of this animal is no laughing matter. (2, 5)
21: Work on a site for Baltic state (7)