Crossword Genius

Furthest former watercourse, we're told? (7)

Ross

I believe the answer is:

extreme

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

'furthest' is the definition.
(synonyms)

'former watercourse we're told?' is the wordplay.
I cannot really understand how this works, but
'former' could be 'ex' (prefix meaning former or onetime) and 'ex' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for extreme that I've seen before include "Most distant" , "not near the centre" , "furthest from the centre" , "not least" , "Very great; exceptional" .)

I've seen this clue in The Telegraph.
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.