More mischievous (9)
I believe the answer is:
naughtier
'more mischievous' is the definition.
('naughty' can be a synonym of 'mischievous')
This is all the clue.
(Other definitions for naughtier that I've seen before include "More badly behaved" , "Less well behaved" , "More mischievous" , "more unruly" , "Worse" .)