Stuns fat South African mountain idiot eating tons (12)

I believe the answer is:

flabbergasts

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'stuns' is the definition.
Although both the answer and definition are verbs in their -s form, I don't understand how one could define the other.

'fat south african mountain idiot eating tons' is the wordplay.
'fat south african mountain' becomes 'flabberg' (I can't justify this - if you can you should give a lot more credence to this answer).
'idiot' becomes 'ass' (both can mean a stupid person).
'eating' means one lot of letters goes inside another.
'tons' becomes 't'.
'ass' placed around 't' is 'asts'.
'flabberg'+'asts'='FLABBERGASTS'

Can you help me to learn more?

(Other definitions for flabbergasts that I've seen before include "staggers!" , "floors" .)

I've seen this clue in The Guardian.

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