Superlative mass reflected cardinal (5)

I believe the answer is:

vital

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

'cardinal' is the definition.
(thesaurus)

'superlative mass reflected' is the wordplay.
'VITAL' is hidden backwards within the letters.
But, I am unsure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for vital that I've seen before include "alive" , "Necessary for life" , "Crucial" , "Urgently needed or absolutely necessary" , "Essential to life or critically important" .)

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

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