Crossword Genius

Formal American word meaning "first" (6)

Ross

I believe the answer is:

primus

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'word meaning first' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'formal american' is the wordplay.
'formal' becomes 'prim' (I've seen this before).
'american' becomes 'us' (abbreviation for United States).
'prim'+'us'='PRIMUS'

Can you help me to learn more?

(Other definitions for primus that I've seen before include "Scottish bishop" , "Camping stove brand" , "Bishop; stove" , "Portable cooking stove for campers" , "cooker" .)

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