Emphatic denial of second person in northern river (5)
I believe the answer is:
never
'emphatic denial' is the definition.
(I've seen this in another clue)
'second person in northern river' is the wordplay.
'second person' becomes 'eve' (biblical second person created after Adam).
'in' means one lot of letters goes inside another.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'river' becomes 'r' (abbreviation for river).
'n'+'r'='nr'
'eve' placed into 'nr' is 'NEVER'.
'of' acts as a link.
(Other definitions for never that I've seen before include "Under no circumstances" , "when pigs fly" , "Surely not" , "Certainly not!" , "Strong negative" .)