Slapped beginner will smart (5)
I believe the answer is:
swish
'smart' is the definition.
(I've seen this before)
'slapped beginner will' is the wordplay.
'beginner' indicates taking the first letters.
'will' becomes 'wish' (synonyms).
The initial letter of 'slapped' is 's'.
's'+'wish'='SWISH'
(Other definitions for swish that I've seen before include "Impressively fashionable" , "Stylish (sl.)" , "Classy" , "Impressively smart" , "Extravagant" .)