Sound of a high-five (4)
I believe the answer is:
slap
'sound of a high-five' 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 "Cuff" , "Blow (in face, on back, wrist)" , "Blow, maybe in the face" , "Right" , "Strike (in the face)" .)