Odd heron in French river (5)
I believe the answer is:
rhone
'french river' is the definition.
(the River Rhone is partly in France)
'odd heron' is the wordplay.
'odd' indicates an anagram.
'heron' with letters rearranged gives 'RHONE'.
'in' acts as a link.
(Other definitions for rhone that I've seen before include "Fr. river and wine" , "River flowing through Lake Geneva" , "French wine region" , "River rising in Switzerland and flowing into the Mediterranean" , "department" .)