Simultaneous discharge of artillery (5)
I believe the answer is:
salvo
'simultaneous discharge of artillery' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for salvo that I've seen before include "Burst of fire from several guns" , "Discharge of gunfire" , "Simultaneous discharge of artillery" , "Burst of firing" , "A simultaneous salute of guns" .)