Friends turn to hit (4)
I believe the answer is:
slap
'to hit' is the definition.
(both can mean to strike)
'friends turn' is the wordplay.
'friends' becomes 'pals' (pal is a kind of friend).
'turn' shows that the letters should be reversed in order.
'pals' backwards is 'SLAP'.
(Other definitions for slap that I've seen before include "Hit with the palm" , "Blow, maybe in the face" , "Make-up (slang)" , "Sharp smack" , "Hit with an open hand" .)