Everybody upset on climbing American plain (5)

I believe the answer is:

llano

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'american plain' is the definition.
'llano' can be an answer for 'plain' (a grassy plain). I am not sure about the 'american' bit.

'everybody upset on climbing' is the wordplay.
'everybody' becomes 'all' (all people).
'upset' shows that the letters should be reversed in order.
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'all' reversed gives 'lla'.
'on' reversed gives 'no'.
'lla'+'no'='LLANO'

Can you help me to learn more?

(Other definitions for llano that I've seen before include "(South American) grassy plain" , "Grassy plain in South?America" , "Treeless area" , "Flat area" , "Treeless grassy plain in South America" .)

I've seen this clue in The Guardian.

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