Battery unit (4)
I believe the answer is:
volt
'battery unit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for volt that I've seen before include "Unit of electric potential difference" , "Electrical SI unit named after a pioneering Italian physicist, d. 1827" , "Electrical unit of measurement" , "little force" , "Unit of electric force" .)