Friends back strike (4)
I believe the answer is:
slap
'strike' is the definition.
(I know that slap is a more specific form of strike)
'friends back' is the wordplay.
'friends' becomes 'pals' (pal is a kind of friend).
'back' says the letters should be written in reverse.
'pals' written backwards gives 'SLAP'.
(Other definitions for slap that I've seen before include "Make-up (slang)" , "Strike (in the face)" , "Cuff" , "Mild punishment" , "Hit with the palm" .)