One river's being diverted, going back to former course? (9)
I believe the answer is:
reversion
'going back to former course?' is the definition.
'one river's being diverted' is the wordplay.
'being diverted' is an anagram indicator (each letter get diverted elsewhere).
'one'+'rivers'='onerivers'
'onerivers' with letters rearranged gives 'REVERSION'.
(Other definitions for reversion that I've seen before include "In right of succession" , "Rightful return of property to owner or heir" .)