Happiest in a chair made of styrene and boxes (9)

I believe the answer is:

sedentary

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

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for sedentary that I've seen before include "not exercising one's talents?" , "Still working" , "Tending to spend too much seated and taking little exercise" , "Yes, red ant could be sitting" , "Sitting much and taking little exercise" .)

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

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