Mad in Parisian river, say (6)
I believe the answer is:
insane
'mad' is the definition.
('insane' can be a synonym of 'mad')
'in parisian river say' is the wordplay.
'parisian river' becomes 'seine' (I've seen this before).
'say' shows a homophone (sound like).
'seine' is a homophone of 'sane'.
'in'+'sane'='INSANE'
(Other definitions for insane that I've seen before include "Wild < cuckoo < cracked" , "Mentally ill" , "nuts" , "Out of one's mind" , "out to lunch" .)