Wake-up call (8)
I believe the answer is:
reveille
'wake-up call' is the definition.
(military wake-up call played on an instrument)
This is all the clue.
(Other definitions for reveille that I've seen before include "Signal to awaken soldiers at daybreak" , "Summons to get up" , "Military signal to wake up" , "Army waking signal" , "Soldiers' wakening call" .)