Noted study of a river (4)
I believe the answer is:
eden
'a river' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'noted study' is the wordplay.
'noted' becomes 'e' (I can't explain this - if you can you should believe this answer much more).
'study' becomes 'den' (term for a study or office).
'e'+'den'='EDEN'
'of' is the link.
Can you help me to learn more?
(Other definitions for eden that I've seen before include "Biblical garden" , "prime minister in the past" , "Paradise - ex-prime minister" , "Someone once seen in Number Ten's" , "heavenly spot" .)