Riverside territory in the country (6)

I believe the answer is:

poland

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'the country' is the definition.
(Poland is an example)

'riverside territory' is the wordplay.
'riverside' becomes 'po' (I can't explain this - if you can you should give a lot more credence to this answer).
'territory' becomes 'land' (land is a kind of territory).
'po'+'land'='POLAND'

'in' is the link.

Can you help me to learn more?

(Other definitions for poland that I've seen before include "Its capital is Warsaw" , "Slovakia's northern neighbour" , "EU member" , "European nation" , "Country joining the European Union in 2004" .)

I've seen this clue in the Evening Standard.

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