Miles commanded English to go for Arthur's traitorous enemy (7)

I believe the answer is:

mordred

I'm a little stuck... Click here to teach me more about this clue!

'arthur's traitorous enemy' is the definition.
(figure of Arthurian legend)

I can't explain the rest of the clue.

(Other definitions for mordred that I've seen before include "knight" , "King Arthur's nephew, who abducted Guinevere" .)

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.