The militant's first summons (8)
I believe the answer is:
reveille
'summons' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for reveille that I've seen before include "Signal to awaken soldiers at daybreak" , "call to start" , "Early call" , "Army waking signal" , "What soldiers get up to" .)