Cheeky whippersnapper — Japan's cake (anag) (10)
I believe the answer is:
jackanapes
(Other definitions for jackanapes that I've seen before include "Cheeky chappie" , "Cheeky sort" , "Cheeky kid - Japan's cake (anag)" , "Conceited, impertinent person" , "Someone unimportant but cheeky and presumptuous" .)