Lucid French river, say (4)
I believe the answer is:
sane
'lucid' is the definition.
('sane' can be similar in meaning to 'lucid')
'french river say' is the wordplay.
'french river' becomes 'seine' (I've seen this before).
'say' shows a homophone (sound like).
'seine' sounds like 'SANE'.
(Other definitions for sane that I've seen before include "Mentally sound" , "Healthy in mind" , "In full possession of mental faculties" , "Normal" , "Identical" .)