Only in sacred mantra, I love traditional Neapolitan song (1,4,3)
I believe the answer is:
o sole mio
'neapolitan song' is the definition.
(I've seen this in another clue)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for o sole mio that I've seen before include "Italian original of Presley's It's Now or Never" , "It's Now or Never used this Italian song" , "Neapolitan song" , "Italian song anglicised as It's Now or Never" , "Air" .)