Artillery bombardment (9)
(Other definitions for cannonade that I've seen before include "Period of continuous heavy gunfire" , "Barrage" , "attack" .)
'artillery bombardment' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for shellfire that I've seen before include "Act of war" , "What generates multiple reports" , "use of artillery" , "Barrage" , "Bombardment from heavy artillery" .)