Smack friends the wrong way (4)
I believe the answer is:
slap
'smack' is the definition.
('slap' can be a synonym of 'smack')
'friends the wrong way' is the wordplay.
'friends' becomes 'pals' (pal is a kind of friend).
'the wrong way' shows that the letters should be reversed in order.
'pals' back-to-front is 'SLAP'.
(Other definitions for slap that I've seen before include "Cuff" , "Mild punishment" , "Blow with open hand" , "Right" , "Sharp smack" .)