Additionally morose? (4,2)

I believe the answer is:

more so

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'additionally morose?' is the definition.
I can't tell whether this defines the answer.

'additionally morose?' is the wordplay.
'morose' can be anagrammed to 'MORE SO'.
Though, I'm not sure how this is indicated.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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