French river, one entering mythology (5)
I believe the answer is:
loire
'french river' is the definition.
(I've seen this before)
'one entering mythology' is the wordplay.
'one' becomes 'i' (Roman numeral).
'entering' is an insertion indicator.
'mythology' becomes 'lore' (I've seen this before).
'i' inserted inside 'lore' is 'LOIRE'.
(Other definitions for loire that I've seen before include "valley famous for its wines" , "department in France" , "Current French" , "flower found amid vineyards" , "flower abroad" .)