Particularly when it’s more than anyone else’s (4,2,3)

I believe the answer is:

most of all

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'else's' is the definition.
Both the definition and answer are adverbs. Maybe there's a link between them I don't understand?

I don't understand the rest of the clue.

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(Other definitions for most of all that I've seen before include "having greatest importance" , "Chiefly" , "Nearly everyone" .)

I've seen this clue in the Evening Standard.

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