Crossword Genius

At its most low, river a nuisance (7)

Ross

I believe the answer is:

deepest

'at its most low' is the definition.

'river a nuisance' is the wordplay.
'river' becomes 'dee' (River Dee**).
'a nuisance' becomes 'pest' (I've seen this before).
'dee'+'pest'='DEEPEST'

(Other definitions for deepest that I've seen before include "most demanding" , "Most profound" , "Profoundest" , "Most bass" , "at the very bottom" .)

I've seen this clue in The Guardian.
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