The little devil's simply without cunning (3)
I believe the answer is:
imp
'the little devil's' is the definition.
(an imp is a little demon)
'simply without cunning' is the wordplay.
'without' is a deletion indicator.
'cunning' becomes 'sly' (synonyms).
'simply' with 'sly' removed is 'IMP'.
(Other definitions for imp that I've seen before include "Malicious spirit" , "Elf" , "old graft" , "Naughty child" , "Little mischief-maker" .)