Legal principle introduced by chancy president (2,4)

I believe the answer is:

cy pres

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'legal principle' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Maybe there's a link between them I don't understand?

'introduced by chancy president' is the wordplay.
'introduced by' indicates a hidden word.
'CY PRES' can be found hidden inside 'chancy president'.

Can you help me to learn more?

(Another definition for cy pres that I've seen is " law".)

I've seen this clue in The Guardian.

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