Six score in a way that is essential (7)

I believe the answer is:

vitally

I'm a little stuck... Click here to teach me more about this clue!

'in a way that is essential' is the definition.
Both the definition and answer are adverbs. Maybe there's an association between them I don't understand?

'six score' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'score' becomes 'tally' (synonyms).
'vi'+'tally'='VITALLY'

Can you help me to learn more?

(Other definitions for vitally that I've seen before include "crucially" , "Essentially" .)

I've seen this clue in the Sydney Morning Herald.

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