Hit mates back (4)
I believe the answer is:
slap
'hit' is the definition.
(both can mean to strike)
'mates back' is the wordplay.
'mates' becomes 'pals' (I've seen this in another clue).
'back' is a reversal indicator.
'pals' back-to-front is 'SLAP'.
(Other definitions for slap that I've seen before include "Hit with an open hand" , "Mild punishment" , "A blow among pals" , "Blow with open hand" , "Right" .)