Trendy officer, as a rule (2,7)
I believe the answer is:
in general
'as a rule' is the definition.
(I've seen this in another clue)
'trendy officer' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'officer' becomes 'general' (general officer is a kind of officer).
'in'+'general'='IN GENERAL'
(Other definitions for in general that I've seen before include "Commonly" , "Overall" , "by and large" , "on the whole" , "More-or-less" .)