Friends returned smack (4)
I believe the answer is:
slap
'smack' is the definition.
(I know that smack can be written as slap)
'friends returned' is the wordplay.
'friends' becomes 'pals' (pal is a kind of friend).
'returned' 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 "Hit with an open hand" , "Make-up (slang)" , "Blow with open hand" , "Mild punishment" , "Sharp smack" .)