Mad character in sight near dishevelled major (15)

I believe the answer is:

earthshattering

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'mad character in sight near dishevelled major' is the definition.
I can't tell whether this definition defines the answer.

'mad character in sight near dishevelled' is the wordplay.
'mad character' becomes 'hatter' (the mad hatter from Alice in Wonderland).
'in' means one lot of letters goes inside another.
'dishevelled' is an anagram indicator.
'sight'+'near'='sightnear'
'sightnear' is an anagram of 'earthsing'.
'hatter' put into 'earthsing' is 'EARTHSHATTERING'.

Can you help me to learn more?

(Another definition for earthshattering that I've seen is " of great importance".)

I've seen this clue in The Daily Express.

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