Sylvester is artful (3)
I believe the answer is:
sly
'artful' is the definition.
(similar in meaning)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for sly that I've seen before include "Doing things in unpleasant and secret way" , "Unpleasantly cunning and secretive" , "Wily, shifty" , "Cunning, deceitful" , "Cunning; surreptitious" .)