Wickedness in tedious mischief (7)
I believe the answer is:
devilry
'wickedness' is the definition.
(I know that devilry is a type of wickedness)
'in tedious mischief' is the wordplay.
'in' indicates putting letters inside.
'tedious' becomes 'dry' (I've seen this in other clues).
'mischief' becomes 'evil' (I've seen this before).
'dry' placed around 'evil' is 'DEVILRY'.
(Other definitions for devilry that I've seen before include "wickedness" , "wicked stuff" , "wicked behaviour" , "reckless mischief?" , "Wicked activity" .)