Case when number one inhabitant amasses millions (10)

I believe the answer is:

nominative

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'amasses millions' is the definition.
I can't judge whether this definition defines the answer.

'case when number one inhabitant' is the wordplay.
'case when' becomes 'nom' (I am not sure about the 'when' bit.).
'number one' becomes 'I' (Roman numeral).
'inhabitant' becomes 'native' (I have seen 'Original inhabitant ' mean 'native' so perhaps 'inhabitant' could also mean 'native').
'nom'+'i'+'native'='NOMINATIVE'

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(Other definitions for nominative that I've seen before include "case for Latin?" , "Appointed" .)

I've seen this clue in The Guardian.

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