River runs in every colour of the rainbow (6)

I believe the answer is:

severn

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

'river' is the definition.
(River Severn)

I can't explain the rest of the clue.

(Other definitions for severn that I've seen before include "River at Worcester" , "Worcester's river" , "River flowing into the Bristol Channel" , "Running water" , "Longest UK river" .)

I've seen this clue in The Times.

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.