Cool river beyond which a castle built (2-8)

I believe the answer is:

de-escalate

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'cool river beyond' is the definition.
I can't tell whether this definition defines the answer.

'which a castle built' is the wordplay.
'which' becomes 'dee' (I can't justify this - if you can you should give a lot more credence to this answer).
'built' indicates anagramming the letters (the same letters can be used to build the answer).
'a'+'castle'='acastle'
'acastle' with letters rearranged gives 'scalate'.
'dee'+'scalate'='DE-ESCALATE'

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I've seen this clue in The Guardian.

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