All sent one quiz, essentially it's boredom (5)
I believe the answer is:
ennui
'it's boredom' is the definition.
(synonyms)
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 "Listless boredom" , "Boredom, tedium" , "doldrums" , "Bored listlessness" , "Sense of tedium" .)