Gun rule read out (6)
I believe the answer is:
cannon
'gun' is the definition.
(cannon is a kind of gun)
'rule read out' is the wordplay.
'rule' becomes 'canon' (canon is a kind of rule).
'read out' indicates a 'sounds like' (homophone) clue.
'canon' sounds like 'CANNON'.
(Other definitions for cannon that I've seen before include "Howitzer" , "Gun sounds like a churchman" , "Old naval gun" , "Caught girl on piece of artillery" , "Weapon firing large balls" .)