Journey starts on time: on up river (4)
I believe the answer is:
tour
'journey' is the definition.
(I know that tour is a more specific form of the action journey)
'starts on time on up river' is the wordplay.
'starts on' indicates taking the first letters.
The first letters of 'time on up river' is 'TOUR'.
(Other definitions for tour that I've seen before include "A trip round" , "Drive" , "Term of duty" , "Go round" , "Extensive journey" .)