Military commander admitting oversight finally — one's fired (7)
I believe the answer is:
shotgun
'one's fired' is the definition.
'military commander admitting oversight finally' is the wordplay.
'military commander' becomes 'shogun' (I've seen this before).
'admitting' indicates putting letters inside.
'finally' suggests the final letters.
The last letter of 'oversight' is 't'.
'shogun' enclosing 't' is 'SHOTGUN'.
(Other definitions for shotgun that I've seen before include "Noughts (anag) - type of wedding" , "Firearm used at short range - kind of wedding" , "piece" , "arm" , "Hunting firearm" .)