Every third Etonian studying shows dissatisfaction (5)
I believe the answer is:
ennui
'dissatisfaction' is the definition.
(ennui is a kind of dissatisfaction)
I don't understand the remainder of the clue.
(Other definitions for ennui that I've seen before include "tiredness" , "Boredom or discontent" , "Bored listlessness" , "Effect of boredom" , "Weary feeling" .)