We’d perished in fog in part of USA (7)

I believe the answer is:

midwest

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'part of usa' is the definition.
('Midwest' is part of 'usa')

I cannot understand how the remainder of the clue works.

(Other definitions for midwest that I've seen before include "American region" , "Area from Ohio to the Rockies" , "north central US" , "Region from Ohio to the Rockies" , "North central region of the United States" .)

I've seen this clue in The Times.

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