Tedium starts to exhaust now nothing usually interests (5)
I believe the answer is:
ennui
'tedium' is the definition.
(I know that tedium can be written as ennui)
'starts to exhaust now nothing usually interests' is the wordplay.
'starts to' indicates taking the first letters.
The first letters of 'exhaust now nothing usually interests' is 'ENNUI'.
(Other definitions for ennui that I've seen before include "Boredom or discontent" , "French boredom and lassitude" , "Listless boredom" , "Feeling when tired of life?" , "Boredom, languor" .)