Turn off none? That's plain (5)

I believe the answer is:

llano

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'that's plain' is the definition.
(a grassy plain)

'turn off none?' is the wordplay.
'turn' says the letters should be written backwards.
'off' becomes 'all' (I can't explain this - if you can you should believe this answer much more).
'none?' becomes 'no'.
'all' backwards is 'lla'.
'lla'+'no'='LLANO'

Can you help me to learn more?

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

I've seen this clue in The Guardian.

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