Learning about Italy's principal river (5)
I believe the answer is:
loire
'river' is the definition.
(Loire is an example)
'learning about italy's principal' is the wordplay.
'learning' becomes 'lore' (I've seen this before).
'about' indicates putting letters inside.
'principal' indicates taking the first letters.
The initial letter of 'italys' is 'i'.
'lore' placed around 'i' is 'LOIRE'.
(Other definitions for loire that I've seen before include "European river" , "Wine region" , "department in France" , "French department" , "Longest French river" .)