Sylvester is briefly crafty (3)
I believe the answer is:
sly
'crafty' is the definition.
(sly can mean crafty or cunning)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sly that I've seen before include "Unpleasantly cunning and secretive" , "'Cunning, wily (3)'" , "Cunning; surreptitious" , "Crafty, devious" , "'Crafty, cunning (3)'" .)