Furthest former watercourse, we're told? (7)
I believe the answer is:
extreme
'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" .)