Lucid French river, say (4)
I believe the answer is:
Here is my best explanation:
'lucid' is the definition.
('sane' can be similar in meaning to 'lucid')
'french river say' is the subsidiary indication.
'french river' becomes 'seine' (I've seen this before).
'say' indicates a 'sounds like' (homophone) clue.
'seine' sounds like 'sane'.
(Other definitions for sane that I've seen before include "Not mad", "Mentally healthy", "In full possession of mental faculties", "Normal", "Mentally sound", "Sound in mind", "Identical", "Rational", "Sensible" and "Free from mental disorder".)