No sailor dies at sea? That’s officially confirmed (9)

I believe the answer is:

notarised

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'officially confirmed' is the definition.
'notarised' can be an answer for 'confirmed' ('confirmed'->'notarized' is in my internal thesaurus and 'ize'->'ise'). I am unsure of the 'officially' bit.

'no sailor dies at sea?' is the wordplay.
'sailor' becomes 'tar' (term for a sailor).
'at sea?' indicates an anagram (I've seen 'at sea' mean this).
'dies' with letters rearranged gives 'ised'.
'no'+'tar'+'ised'='NOTARISED'

'that's' acts as a link.

Can you help me to learn more?

(Another definition for notarised that I've seen is " Authenticated".)

I've seen this clue in The Times.

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