Commandos' leader soon to impound new gun (6)
I believe the answer is:
cannon
'gun' is the definition.
(cannon is a kind of gun)
'commandos' leader soon to impound new' is the wordplay.
'leader' says to take the initial letters.
'soon' becomes 'anon' (I've seen this before).
'to impound' is an insertion indicator.
'new' becomes 'n'.
The initial letter of 'commandos' is 'c'.
'anon' enclosing 'n' is 'annon'.
'c'+'annon'='CANNON'
(Other definitions for cannon that I've seen before include "Heavy ordnance" , "The large gun sounds like a church dignitary" , "shot playing billiards" , "Weapon firing large balls" , "Howitzer" .)