What might give heart a big shock?

I believe the answer is:

earth-shattering

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

'what might give heart a big shock?' is the definition.
I can't judge whether this defines the answer.

'what might give heart' is the wordplay.
I cannot quite understand 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 see.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Telegraph.

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