Mischief-maker is simply without cunning (3)
I believe the answer is:
imp
'mischief-maker' is the definition.
(mischievous creature)
'simply without cunning' is the wordplay.
'without' is a deletion indicator.
'cunning' becomes 'sly' (synonyms).
'simply' with 'sly' removed is 'IMP'.
'is' acts as a link.
(Other definitions for imp that I've seen before include "Rascal" , "Mischievous creature" , "Mischievous fairy No.1" , "pickle" , "Mischievous monkey" .)