River, one featuring in folk legend (5)
I believe the answer is:
loire
'river' is the definition.
(I know that Loire is a river)
'one featuring in folk legend' is the wordplay.
'one' becomes 'i' (Roman numeral).
'featuring in' indicates putting letters inside.
'folk legend' becomes 'lore' (I've seen this in another clue).
'i' put into 'lore' is 'LOIRE'.
(Other definitions for loire that I've seen before include "flower abroad" , "Current French" , "banker" , "flower found amid vineyards" , "River flowing through Tours and Nantes" .)