Artillery units providing power (9)
I believe the answer is:
batteries
'artillery units' is the definition.
(battery is a kind of artillery unit)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for batteries that I've seen before include "They convert chemical energy into electrical energy" , "Containers with cells producing power" , "Portable energy sources" , "Electric cells supplying current" , "Devices with cells that produce electricity" .)