Tradition about one river (5)
I believe the answer is:
loire
'river' is the definition.
(Loire is an example)
'tradition about one' is the wordplay.
'tradition' becomes 'lore' (I've seen this in another clue).
'about' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'lore' going around 'i' is 'LOIRE'.
(Other definitions for loire that I've seen before include "France's longest river" , "French department" , "French river (Orleans, Tours, Nantes)" , "Wine region" , "European river" .)