R' for 'run-of-the-mill (6-2-3-4)

I believe the answer is:

middle-of-the-road

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

'run-of-the-mill' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for middle-of-the-road that I've seen before include "place for dangerous drivers?" , "Avoiding extremes; moderate" , "Average" , "Unexciting" , "Safe" .)

I've seen this clue in The Independent.

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.