Crazy to be in French river, say (6)

I believe the answer is:

insane

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'crazy' is the definition.
(similar in meaning)

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

'to be' is the link.

Can you help me to learn more?

(Other definitions for insane that I've seen before include "having a screw loose" , "Sienna (anag.)" , "off trolley" , "reckless" , "Anne is (anag) - crazy" .)

I've seen this clue in The Mirror.

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