Poem on river — European one (4)
I believe the answer is:
oder
'european one' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'poem on river' is the wordplay.
'poem' becomes 'ode' (ode is a kind of poem).
'on' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation for river).
'ode'+'r'='ODER'
(Other definitions for oder that I've seen before include "European flower" , "waterway" , "flowing water" , "German and Polish river" , "German banker" .)