Top officer over in DEFCON reportedly avoided rules (4,2,7)
I believe the answer is:
code of conduct
'rules' is the definition.
(a set of rules for behaviour)
'top officer over in defcon reportedly avoided' is the wordplay.
'top officer' becomes 'CO' (commanding officer).
'over' becomes 'o' (cricket abbreviation).
'in' indicates putting letters inside.
'reportedly' shows a homophone (sound like).
'avoided' becomes 'ducked' (duck can mean to avoid).
'ducked' sounds like 'duct'.
'o' placed within 'defcon' is 'deofcon'.
'co'+'deofcon'+'duct'='CODE OF CONDUCT'
(Other definitions for code of conduct that I've seen before include "Established set of rules for behaviour" , "Established rules for dealing with particular situations" .)