Crossword Genius

Immaculate as Swiss cheese, we hear? (4)

Ross

I believe the answer is:

holy

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'immaculate' is the definition.
(thesaurus)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for holy that I've seen before include "Sacred" , "perfect, morally and spiritually" , "Charlemagne was the first" , "Dedicated to God" , "Hallowed" .)

I've seen this clue in The Guardian.
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