Essentially unmuzzle his gun (3)
I believe the answer is:
uzi
'gun' is the definition.
(military weapon)
'essentially unmuzzle his' is the wordplay.
'essentially' indicates the central letters.
'his' becomes 'i' (I can't explain this - if you can you should give a lot more credence to this answer).
The middle of 'unmuzzle' is 'uz'.
'uz'+'i'='UZI'
Can you help me to learn more?
(Other definitions for uzi that I've seen before include "Submachine-gun" , "Israeli-designed sub-machine gun" , "kind of weapon" , "killer" , "Israeli sub-machine gun" .)