Tug used in river flood (6)
I believe the answer is:
deluge
'flood' is the definition.
('deluge' can be a synonym of 'flood')
'tug used in river' is the wordplay.
'tug' becomes 'lug' ('lug' can be a synonym of 'tug').
'used in' is an insertion indicator.
'river' becomes 'dee' (River Dee).
'lug' placed inside 'dee' is 'DELUGE'.
(Other definitions for deluge that I've seen before include "Cloudburst, downpour" , "Inundation" , "Major flood" , "Severe flood" , "Torrential rain" .)