At its most low, river a nuisance (7)
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" .)