One girl to upset, being a fool (5)

I believe the answer is:

idiot

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'being a fool' is the definition.
(I've seen this before)

'one girl to upset' is the wordplay.
'one girl' becomes 'idi' (I can't justify this - if you can you should believe this answer much more).
'upset' shows that the letters should be reversed in order.
'to' written backwards gives 'ot'.
'idi'+'ot'='IDIOT'

Can you help me to learn more?

(Other definitions for idiot that I've seen before include "Dolt" , "Silly ass" , "Ass, ninny" , "Stupid person" , "The - (Dostoievsky)" .)

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