Open-handed smack (4)
I believe the answer is:
slap
'open-handed smack' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for slap that I've seen before include "Blow (in face, on back, wrist)" , "Strike with the palm" , "A blow among pals" , "Hit with the palm" , "Blow, maybe in the face" .)