Really enjoyed this one. Thanks for all the tips in parsing. Like Phil, I didn't have the option of Latin at school, so couldn't parse 3d either. Also couldn't parse last 4 letters of 21, which was my LOI, as I was fixated on the artist part being RA, thinking the definition was 'not unknown' or 'unknown'. Good misdirection!
Like others, hard to choose one COD, but thought 8 and 13 very clever, and really enjoyed 14, 17, 19 and 20.