Only in sacred mantra, I love traditional Neapolitan song (1,4,3)

I believe the answer is:

o sole mio

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'neapolitan song' is the definition.
(I've seen this in another clue)

I can't explain the rest of the clue.

(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" .)

I've seen this clue in The Guardian.

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