Keeper 002: In Flodden Field

This puzzle is a more straight-faced effort. But, as with my first crossword, there's something of a theme.

Hint: The theme involves two parts of a literary quote (and its source), which might provide a description of the art of setting cryptic crosswords.

12/7 Update: I have revised this puzzle based on some very helpful feedback (Thanks to John and Andy!). Most of the changes were minor (adding, subtracting, or changing a word in a clue), but I did rewrite a few clues, and I actually changed two solutions (7ac and 8dn).