Assume it's the river (3)
I believe the answer is:
don
This clue is a double definition.
'assume' is the first definition.
(thesaurus)
'the river' is the second definition.
(Don is an example)
'it's' acts as a link.
(Other definitions for don that I've seen before include "Put on for the Mafia chief and Spanish aristocrat" , "Get into" , "A Mafia chief like Corleone" , "Slip into" , "Put on the learned man" .)