Crossword Genius

Momentous heart (5-10)

Ross

I believe the answer is:

earth-shattering

I'm a little stuck... Click here to teach me more about this clue!

'momentous' is the definition.
(I've seen this before)

'heart' is the wordplay.
I cannot really see how this works, but
an anagram of 'heart' is 'earth' which is present in the answer.
The remaining letters 'shattering' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.
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