Knock pals back (4)
I believe the answer is:
slap
'knock' is the definition.
(thesaurus)
'pals back' is the wordplay.
'back' says the letters should be written in reverse.
'pals' reversed gives 'SLAP'.
(Other definitions for slap that I've seen before include "Strike with the palm" , "Hit with an open hand" , "Hit with a flat hand" , "Cuff" , "Blow with open hand" .)