Risk coming over river (9)
I believe the answer is:
adventure
'risk' is the definition.
(adventuring is a kind of risking)
'coming over river' is the wordplay.
'coming' becomes 'advent' (advent is the arrival or coming of something).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'river' becomes 'ure' (River Ure).
'advent'+'ure'='ADVENTURE'
(Other definitions for adventure that I've seen before include "Escapade" , "Exciting series of events" , "Risky or exciting experience" , "Exciting or dangerous journey" , "Dangerous undertaking" .)