Discharge from the army (7)
'discharge' is the definition.
(thesaurus)
'the army' is the wordplay.
I cannot quite see how this works, but
'army' could be 're' (Royal Engineers army regiment) and 're' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'from' acts as a link.
Can you help me to learn more?
(Other definitions for gunfire that I've seen before include "Hail maybe" , "Sound of. eg. rifles" , "Sound of shooting" , "Reports" , "Fine rug (anag.)" .)
(Other definitions for demob that I've seen before include "Discharge soldiers (abbr.)" , "Briefly disband" , "Release from armed forces" , "removal from service" , "Discharge from army" .)