Disturbed advisors certify not one of things that philatelists seek (5-3,6)

I believe the answer is:

first-day covers

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'things that philatelists seek' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'disturbed advisors certify not one' is the wordplay.
'disturbed' indicates anagramming the letters.
'not' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'advisors'+'certify'='advisorscertify'
'advisorscertify' with 'i' taken away is 'advsorscertify'.
'advsorscertify' anagrammed gives 'FIRST-DAY COVERS'.

'of' acts as a link.

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I've seen this clue in The Hindu.

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