Not of sound mind in said French river (6)
I believe the answer is:
insane
'not of sound mind' is the definition.
(I've seen this in another clue)
'in said french river' is the wordplay.
'said french river' becomes 'sane' (I can't justify this - if you can you should believe this answer much more).
'in'+'sane'='INSANE'
Can you help me to learn more?
(Other definitions for insane that I've seen before include "Mad, irrational" , "Demented" , "nuts" , "Anne is (anag) - crazy" , "out to lunch" .)