Tedium something of an unspoken nuisance (5)
I believe the answer is:
ennui
'tedium' is the definition.
(I know that tedium can be written as ennui)
'something of an unspoken nuisance' is the wordplay.
'something of' says the answer is hidden in the clue (I've seen 'something' mean this (I've seen this in other clues)).
'ENNUI' is hidden within 'unspoken nuisance'.
(Other definitions for ennui that I've seen before include "bore" , "Listless feeling" , "Listless boredom" , "French boredom and lassitude" , "Feeling of disaffection" .)