Drag into a river in flood (6)
I believe the answer is:
deluge
'flood' is the definition.
('deluge' can be a synonym of 'flood')
'drag into a river' is the wordplay.
'drag' becomes 'lug' (I've seen this in other clues).
'into' is an insertion indicator.
'a river' becomes 'dee' (River Dee).
'lug' placed within 'dee' is 'DELUGE'.
'in' is the link.
(Other definitions for deluge that I've seen before include "Flooding downpour" , "Major flood" , "Torrential rain" , "A heavy fall of rain (6)" , "Severe flood" .)