Mad in French river, say (6)
I believe the answer is:
insane
'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" .)