They supply power for gun emplacements (9)
I believe the answer is:
batteries
'emplacements' is the definition.
The definition and answer can be both acts as well as being plural nouns.
Perhaps you can see a link between them that I don't see?
I cannot understand how the rest 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" , "Electric cells supplying current" , "Portable energy sources" , "Containers with cells producing power" , "Devices with cells that produce electricity" .)