Hit friend's back (4)
I believe the answer is:
slap
'hit' is the definition.
(both can mean to strike)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for slap that I've seen before include "Blow with open hand" , "Make-up (slang)" , "Hit with the palm" , "Right" , "Mild punishment" .)