How one spent holidays - or half of them possibly (4)

I believe the answer is:

idly

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'how' is the definition.
Both the definition and answer are adverbs. Perhaps they are linked in a way I don't understand?

I don't understand the remainder of the clue.

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(Other definitions for idly that I've seen before include "How one may stand by" , "How one stands by?" , "In a lazy fashion" , "In a lazy manner" , "without purpose" .)

I've seen this clue in the Evening Standard.

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