Level for one river entering north-western estuary (6)
I believe the answer is:
degree
'level' is the definition.
(I've seen this before)
'for one river entering north-western estuary' is the wordplay.
'for one' becomes 'eg' (I've seen this in other clues).
'river' becomes 'r'.
'entering' indicates putting letters inside.
'north-western estuary' becomes 'dee'.
'eg'+'r'='egr'
'egr' put within 'dee' is 'DEGREE'.
(Other definitions for degree that I've seen before include "Academic award - stage on a scale" , "Academic award - 60 minutes" , "Comparative amount" , "Fraction of a minute" , "Academic qualification" .)