Loud kiss (5)
'loud kiss' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for smack that I've seen before include "Sharp slap" , "Loud kiss" , "Hit with an open hand" , "Heroin (sl.)" , "Hit - fishing vessel" .)
(Another definition for smackeroo that I've seen is " dollar".)