Crossword Genius

Mad in French river, say (6)

Ross

I believe the answer is:

insane

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'mad' is the definition.
(I know that mad can be written as insane)

'in french river say' is the wordplay.
'french river say' becomes 'sane' (I can't justify this - if you can you should give a lot more credence to this answer).
'in'+'sane'='INSANE'

Can you help me to learn more?

(Other definitions for insane that I've seen before include "without any reason" , "Sienna (anag.)" , "off trolley" , "Stupid" , "Eg, gun or knife" .)

I've seen this clue in The Mirror.
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