Mad within Parisian river, say (6)
I believe the answer is:
insane
'mad' is the definition.
(I know that mad can be written as insane)
'within parisian river say' is the wordplay.
'within' becomes 'in'.
'parisian river' becomes 'seine' (I've seen this in another clue).
'say' indicates a 'sounds like' (homophone) clue.
'seine' sounds like 'sane'.
'in'+'sane'='INSANE'
(Other definitions for insane that I've seen before include "Stupid" , "Eg, gun or knife" , "nuts" , "out to lunch" , "reckless" .)