A goddess protecting river and lake near Naples (7)
I believe the answer is:
'lake near naples' is the definition.
(Lake Avernus is a crater lake near Naples)
'a goddess protecting river' is the wordplay.
'goddess' becomes 'venus' (Roman goddess of love).
'protecting' means one lot of letters goes inside another (inserted letters are held or protected).
'river' becomes 'r' (abbreviation for river).
'venus' enclosing 'r' is 'vernus'.
'and' acts as a link.
(Other definitions for avernus that I've seen before include "Introduction to underworld" , "Infernal lake" , "Italian lake once thought the entrance to Hades" , "Site in Italy once believed the entrance to Hell" , "Underworld entrance" .)