Copper sheathes this rod used in battery? (9)
I believe the answer is:
Here is my best explanation:
'copper sheathes this rod used in battery?' is the definition.
'truncheon' can be an answer for 'rod' (thesaurus). I am not sure about the rest of the definition.
This is the entire clue.
(Other definitions for truncheon that I've seen before include "Policeman's traditional weapon", "Club", "Police staff", "Baton", "Police baton", "Staff", "Policeman's club", "Police weapon" and "Peelers' baton".)