Crossword Genius

Part of the USA when the mineral wealth's largely been removed (6)

Ross

I believe the answer is:

oregon

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'mineral wealth's largely been removed' is the definition.
I can't judge whether this definition defines the answer.

'part of the usa when' is the wordplay.
'part' says to take the initial letters.
'the usa' becomes 'reg' (I can't explain this - if you can you should believe this answer much more).
'when' becomes 'on' (I've seen this in other clues).
The initial letter of 'of' is 'o'.
'o'+'reg'+'on'='OREGON'

'the' is the link.

Can you help me to learn more?

(Other definitions for oregon that I've seen before include "US state, capital Salem" , "No gore seen in this American state" , "No gore in this western American state" , "Part of the US" , "area of the US" .)

I've seen this clue in the Evening Standard.
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