Flog back to pieces (5)
I believe the answer is:
strap
'flog' is the definition.
(strap can mean to beat or flog)
'back to pieces' is the wordplay.
'back' shows that the letters should be reversed in order.
'to pieces' becomes 'parts' ('part' can be a synonym of 'piece').
'parts' in reverse letter order is 'STRAP'.
(Other definitions for strap that I've seen before include "Secure" , "Fastening strip" , "Parts of a long piece of leather" , "Security device" , "Leather fastening" .)