Eden river, titularly? (4)

I believe the answer is:

avon

I'm a little stuck... Click here to teach me more about this clue!

'eden river' is the definition.
'avon' can be an answer for 'river' (I've seen this before). I am not sure about the 'eden' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for avon that I've seen before include "Name of three English rivers" , "Former English county" , "English river, flows through Stratford" , "River of the Bard" , "River and former county" .)

I've seen this clue in the Evening Standard.

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