No record is more jealously guarded than this (3-6,4)

I believe the answer is:

top-secret file

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'more jealously guarded than this' is the definition.
I know nothing about this answer so I can't tell whether this works.

'no record' is the wordplay.
I cannot quite see how this works, but
'no' could be 'o' (O resembles 0) and 'o' is found within the answer.
'record' could be 'file' (filing is a kind of recording) and 'file' is present in the answer.
This explanation may well be incorrect...

'is' acts as a link.

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I've seen this clue in The Globe and Mail.

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