Copper's weapon rates badly (5)
I believe the answer is:
taser
'copper's weapon' is the definition.
'taser' can be an answer for 'weapon' (I've seen this before). I'm unsure of the 'copper's' bit.
'rates badly' is the wordplay.
'badly' indicates anagramming the letters.
'rates' with letters rearranged gives 'TASER'.
Can you help me to learn more?
(Other definitions for taser that I've seen before include "Stun gun" , "Shocking device used by the police?" , "police weapon introduced" , "Gun firing electrified barbs" , "A shocking thing" .)