Learning about Italy's foremost river (5)
I believe the answer is:
loire
'river' is the definition.
(I know that Loire is a river)
'learning about italy's foremost' is the wordplay.
'learning' becomes 'lore' (both can mean tradition or knowledge).
'about' means one lot of letters goes inside another.
'foremost' indicates taking the first letters.
The first letter of 'italys' is 'i'.
'lore' going around 'i' is 'LOIRE'.
(Other definitions for loire that I've seen before include "valley famous for its wines" , "French runner" , "department in France" , "banker" , "Longest river in France" .)