French river, one in mythology (5)
I believe the answer is:
loire
'french river' is the definition.
(I've seen this before)
'one in mythology' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'mythology' becomes 'lore' (I've seen this before).
'i' put within 'lore' is 'LOIRE'.
(Other definitions for loire that I've seen before include "flower abroad" , "Longest river in France" , "banker" , "French river (Orleans, Tours, Nantes)" , "part of France" .)