Surge of rising river essentially overwhelming (6)
I believe the answer is:
seethe
'surge' is the definition.
(thesaurus)
'rising river essentially overwhelming' is the wordplay.
'rising' says the letters should be written in reverse (going up in a down clue).
'river' becomes 'tees' (River Tees**).
'essentially' means to look at the middle letters.
The central letters of 'overwhelming' are 'he'.
'tees' in reverse letter order is 'seet'.
'seet'+'he'='SEETHE'
'of' acts as a link.
(Other definitions for seethe that I've seen before include "Boil turbulently" , "Smoulder" , "(Of liquid) move as if boiling" , "Bubble gently like boiling water" , "Simmer with anger" .)