River dweller in countrywide draw (8,7)

I believe the answer is:

national lottery

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'draw' is the definition.
(UK prize draw)

'river dweller in countrywide' is the wordplay.
'river dweller' becomes 'otter' (I am not sure about this - if you are sure you should believe this answer much more).
'in' means one lot of letters goes inside another.
'countrywide' becomes 'nationally'.
'otter' going inside 'nationally' is 'NATIONAL LOTTERY'.

Can you help me to learn more?

(Other definitions for national lottery that I've seen before include "Game with very little chance of winning" , "sponsor of good causes" , "A gamble" , "fundraiser" , "that country plays for a draw" .)

I've seen this clue in The Guardian.

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