Crossword Genius

Really big or ground-breaking (5-10)

Ross

I believe the answer is:

earth-shattering

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

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

'ground-breaking' is the wordplay.
I cannot really understand how this works, but
'ground' could be 'earth' (synonyms) and 'earth' is found within the answer.
'breaking' could be 'shattering' (shattering is a kind of breaking) and 'shattering' is located in the answer.
No letters remain.
This explanation may well be incorrect...

'or' acts as a link.

Can you help me to learn more?

I've seen this clue in The Sunday Times.
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