Origin of river's rising (4)
I believe the answer is:
seed
'origin' is the definition.
(seed is a kind of origin)
'river's rising' is the wordplay.
'river's' becomes 'dee's' (River Dee).
'rising' shows that the letters should be reversed in order (in a down clue, letters go up).
'dees' backwards is 'SEED'.
'of' acts as a link.
(Other definitions for seed that I've seen before include "It can develop into a plant" , "An outstanding player in a tournament" , "- Pearl; - capital" , "Pip for sowing" , "Pip; origin, beginning" .)