How to spend a solidly sunless day off work (4)

I believe the answer is:

idly

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

'spend a solidly sunless day off work' is the wordplay.
'IDLY' is hidden in amongst the letters.
However, I'm unsure how this is indicated.

'to' is the link.

Can you help me to learn more?

(Other definitions for idly that I've seen before include "without thinking" , "without good reason" , "In a lazy way" , "So lacking purpose" , "in a way vain" .)

I've seen this clue in the Evening Standard.

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