Sensible French river report (4)
I believe the answer is:
sane
'sensible' is the definition.
(sane can mean sensible or reasonable)
'french river report' is the wordplay.
'french river' becomes 'seine' (I've seen this before).
'report' indicates a 'sounds like' (homophone) clue.
'seine' is a homophone of 'SANE'.
(Other definitions for sane that I've seen before include "Compos mentis" , "Sound in mind" , "Not mad" , "Reasonable" , "Identical" .)