One may waltz about in European river (6)
I believe the answer is:
dancer
'one' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'may waltz about in european river' is the wordplay.
'may waltz about' becomes 'c' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'european' becomes 'Dane' (someone from Denmark).
'river' becomes 'r' (abbreviation for river).
'c' inserted within 'dane' is 'dance'.
'dance'+'r'='DANCER'
Can you help me to learn more?
(Other definitions for dancer that I've seen before include "See 25" , "Craned (anag.)" , "Entertainer" , "Mover to music" , "Ballet performer" .)