Discharge from the artillery (5)
I believe the answer is:
salvo
'discharge' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for salvo that I've seen before include "Ceremonial shots" , "Sudden round of applause" , "reservation" , "Unusual excuse" , "Round of fire" .)