Sent out on river journey (4)
I believe the answer is:
tour
'journey' is the definition.
(touring is a kind of journeying)
'sent out on river' is the wordplay.
'sent' is an anagram indicator (send can mean to excite or stimulate).
'on' says to put letters next to each other.
'river' becomes 'r'.
'out' anagrammed gives 'tou'.
'tou'+'r'='TOUR'
(Other definitions for tour that I've seen before include "Extended journey" , "Travel around" , "Short trip; spell of duty" , "Trip, visit" , "Extensive journey" .)