Tug sunk in river flood (6)
I believe the answer is:
deluge
'flood' is the definition.
('deluge' can be a synonym of 'flood')
'tug sunk in river' is the wordplay.
'tug' becomes 'lug' ('lug' can be a synonym of 'tug').
'sunk in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'lug' inserted into 'dee' is 'DELUGE'.
(Other definitions for deluge that I've seen before include "Major flood" , "Flood - downpour" , "Torrential rain" , "Inundation" , "Severe flood" .)