Convince Jenny to go by river (6)
I believe the answer is:
assure
'convince' is the definition.
('assure' can be a synonym of 'convince')
'jenny to go by river' is the wordplay.
'jenny' becomes 'ass' (jenny is a female ass).
'to go by' is a charade indicator (letters next to each other).
'river' becomes 'ure' (I have seen 'River of Wensleydale ' mean 'ure' so perhaps 'river' could also mean 'ure').
'ass'+'ure'='ASSURE'
(Other definitions for assure that I've seen before include "Tell positively" , "Make sure, guarantee" , "State positively and confidently, guarantee" , "Convince, satisfy" , "Cover by life insurance" .)