Crossword Genius

Letter, one about hidden region of China (5)

Ross

I believe the answer is:

macau

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

'region of china' is the definition.
(I've seen this before)

'letter one about hidden' is the wordplay.
I cannot really see how this works, but
'letter' could be 'u' and 'u' is found in the answer.
'one' could be 'a' (a thing is one thing) and 'a' is located in the answer.
'about' could be 'ca' (circa) and 'ca' is found within the answer.
A single letter 'm' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for macau that I've seen before include "region of China" , "Part of China" , "SW of 24 [SHANGHAI]" , "Small peninsula in China, a Portuguese territory until 1999" , "somewhere in China" .)

I've seen this clue in The Guardian.
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