Rascally as Jack? (7)
I believe the answer is:
knavish
'rascally' is the definition.
(thesaurus)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for knavish that I've seen before include "Villainous" , "Rascally" , "Dishonest, unscrupulous" , "Wily" , "Like unscrupulous man" .)