Boredom in kindergarten -- nuisance! (5)
I believe the answer is:
ennui
'boredom' is the definition.
('ennui' can be a synonym of 'boredom')
'in kindergarten nuisance' is the wordplay.
'in' indicates a hidden word.
'ENNUI' is hidden in the letters of 'kindergarten nuisance'.
(Other definitions for ennui that I've seen before include "Boredom, dullness" , "French boredom and lassitude" , "bore" , "World-weariness" , "What's tedious and instils such listlessness" .)