Crossword Genius

Hymn producing historic shock? (4,2,4)

Ross

I believe the answer is:

rock of ages

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'hymn' is the definition.
(Christian hymn)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for rock of ages that I've seen before include "Hymn" , "song" .)

I've seen this clue in The Times.
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