Rule about new gun (6)
I believe the answer is:
cannon
'gun' is the definition.
(cannon is a kind of gun)
'rule about new' is the wordplay.
'rule' becomes 'canon' (canon is a kind of rule).
'about' indicates putting letters inside.
'new' becomes 'n'.
'canon' going around 'n' is 'CANNON'.
(Other definitions for cannon that I've seen before include "Heavy ordnance" , "One of the big guns" , "Gun sounds like a churchman" , "Large gun mounted on wheels" , "Large artillery piece" .)