Roderick's river (4)

I believe the answer is:

oder

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

'river' is the definition.
(River Oder in central Europe)

'roderick's' is the wordplay.
'ODER' is hidden in amongst the letters.
However, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for oder that I've seen before include "River on border of Poland and Germany" , "German and Polish river" , "Rode around the European river" , "waterway" , "German river that empties into the Baltic Sea" .)

I've seen this clue in the Evening Standard.

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