The sound of a French river - that's reasonable (4)
I believe the answer is:
sane
'reasonable' is the definition.
(I know that reasonable can be written as sane)
'the sound of a french river' is the wordplay.
'the sound of' shows a homophone (sound like).
'a french river' becomes 'seine' (I've seen this before).
'seine' sounds like 'SANE'.
'that's' acts as a link.
(Other definitions for sane that I've seen before include "Mentally healthy" , "Healthy in mind" , "Mentally sound" , "Not mad" , "In full possession of mental faculties" .)