Tedium encapsulated by outspoken nuisance (5)
I believe the answer is:
ennui
'tedium' is the definition.
(synonyms)
'encapsulated by outspoken nuisance' is the wordplay.
'encapsulated by' says the answer is hidden in the clue (the clue encapsulates or contains the answer).
'ENNUI' can be found hidden inside 'outspoken nuisance'.
(Other definitions for ennui that I've seen before include "Weary feeling" , "Sense of tedium" , "Feeling when tired of life?" , "Listless feeling" , "Mental weariness" .)