Tedium of wasting sunnier spells inside (5)

I believe the answer is:

ennui

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'tedium' is the definition.
(I know that tedium can be written as ennui)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for ennui that I've seen before include "bore" , "Feeling of weariness or boredom" , "Effect of boredom" , "tiredness" , "Boredom in France" .)

I've seen this clue in the Sydney Morning Herald.

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