French river, one embodied in folk tradition (5)
I believe the answer is:
loire
'french river' is the definition.
(River Loire)
'one embodied in folk tradition' is the wordplay.
'one' becomes 'I' (Roman numeral).
'embodied in' means one lot of letters goes inside another.
'folk tradition' becomes 'lore' (lore is traditional knowledge or learning).
'i' going within 'lore' is 'LOIRE'.
(Other definitions for loire that I've seen before include "banker" , "France's longest river" , "valley famous for its wines" , "flower abroad" , "Current French" .)