More mischievous (9)
I believe the answer is:
naughtier
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">more mischievous</span>' is the definition.<br/ ><span class="explanation-format__explanation">('naughty' can be a synonym of 'mischievous')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for naughtier that I've seen before include "More badly behaved" , "Less well behaved" , "More mischievous" , "more unruly" , "Worse" .)