Drag through river in downpour (6)
I believe the answer is:
deluge
'downpour' is the definition.
('deluge' can be a synonym of 'downpour')
'drag through river' is the wordplay.
'drag' becomes 'lug' (I've seen this in other clues).
'through' indicates putting letters inside (I've seen this in other clues).
'river' becomes 'dee' (River Dee).
'lug' put into 'dee' is 'DELUGE'.
'in' is the link.
(Other definitions for deluge that I've seen before include "Cloudburst, downpour" , "A heavy fall of rain (6)" , "Flooding downpour" , "Severe flood" , "Swamp" .)