Heart-broken, make a confession about mad character of tremendous importance (5-10)

I believe the answer is:

earth-shattering

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

'tremendous importance' is the definition.
'earth shattering' can be an answer for 'importance' (I have seen 'great importance' mean 'earth shattering' so perhaps 'importance' could also mean 'earth shattering'). I'm not sure about the 'tremendous' bit.

'heart-broken make a confession about mad character' is the wordplay.
'broken' indicates anagramming the letters.
'make a confession' becomes 'sing' (I can't explain this - if you can you should believe this answer much more).
'about' means one lot of letters goes inside another.
'mad character' becomes 'hatter' (Alice In Wonderland).
'heart' is an anagram of 'earth'.
'earth'+'sing'='earthsing'
'earthsing' enclosing 'hatter' is 'EARTH-SHATTERING'.

'of' acts as a link.

Can you help me to learn more?

I've seen this clue in The Guardian.

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