Learning about one French river (5)
I believe the answer is:
loire
'french river' is the definition.
(I've seen this before)
'learning about one' is the wordplay.
'learning' becomes 'lore' (I've seen this before).
'about' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'lore' placed around 'i' is 'LOIRE'.
(Other definitions for loire that I've seen before include "banker" , "part of France" , "valley famous for its wines" , "Wine region" , "European river" .)