Assume it's the river (3)
I believe the answer is:
don
I believe this clue is a double definition.
'assume' is the first definition.
(I've seen this before)
'the river' is the second definition.
(Yorkshire river)
'it's' is the 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" .)