Tug overwhelmed by river in flood (6)
I believe the answer is:
deluge
'flood' is the definition.
('deluge' can be a synonym of 'flood')
'tug overwhelmed by river' is the wordplay.
'tug' becomes 'lug' ('lug' can be a synonym of 'tug').
'overwhelmed by' indicates putting letters inside (I've seen this in other clues).
'river' becomes 'dee' (River Dee).
'lug' put within 'dee' is 'DELUGE'.
'in' is the link.
(Other definitions for deluge that I've seen before include "Great flood" , "Severe flood" , "Huge downpour" , "Swamp" , "Inundation" .)