False beliefs of French in souls transformed (9)

I believe the answer is:

delusions

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'false beliefs' is the definition.
The answer and definition can be both related to thinking as well as being plural nouns.
Maybe there's a link between them I don't understand?

'of french in souls transformed' is the wordplay.
'of french' becomes 'de'.
'transformed' indicates anagramming the letters.
'in'+'souls'='insouls'
'insouls' with letters rearranged gives 'lusions'.
'de'+'lusions'='DELUSIONS'

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(Another definition for delusions that I've seen is " Symptoms of mental disorder".)

I've seen this clue in The Telegraph.

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