Grand old swindler, clever Geordie? (4)

I believe the answer is:

gleg

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'grand' is the definition.
Both the definition and answer are adjectives.
Perhaps there's an association between them I don't understand?

I can't explain the rest of the clue.

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(Another definition for gleg that I've seen is " Scots keen".)

I've seen this clue in The Times and The Sunday Times.

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