Hit copper with repeated force (4)
I believe the answer is:
cuff
'hit' is the definition.
(I've seen this before)
'copper with repeated force' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'with' is a charade indicator (letters next to each other).
'repeated' indicates the doubling of some letters.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'f' doubled is 'ff'.
'cu'+'ff'='CUFF'
(Other definitions for cuff that I've seen before include "Strike" , "Smack; Moonstone sergeant" , "Blow - on the wrist?" , "Light blow with the open hand" , "Blow; end of sleeve" .)