Strike, but mates go back (4)
I believe the answer is:
slap
'strike' is the definition.
(I know that slap is a more specific form of the action strike)
'mates go back' is the wordplay.
'mates' becomes 'pals' (I've seen this in another clue).
'go back' is a reversal indicator.
'pals' in reverse letter order is 'SLAP'.
'but' is the link.
(Other definitions for slap that I've seen before include "Right" , "Blow with open hand" , "Cuff" , "Strike with the palm" , "Hit with an open hand" .)