Sensible French river, say (4)
I believe the answer is:
sane
'sensible' is the definition.
(sane can mean sensible or reasonable)
'french river say' is the wordplay.
'french river' becomes 'seine' (I've seen this in another clue).
'say' indicates a 'sounds like' (homophone) clue.
'seine' is a homophone of 'SANE'.
(Other definitions for sane that I've seen before include "Mentally healthy" , "Right-minded" , "Mentally sound" , "Compos mentis" , "Normal" .)