Happiest in a chair made of styrene and boxes (9)
I believe the answer is:
sedentary
'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" .)