Mainly popular officer! (2,7)
I believe the answer is:
in general
'mainly' is the definition.
(I've seen this before)
'popular officer' is the wordplay.
'popular' becomes 'in' (something that's 'in' is 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" , "on the whole" , "Overall" , "Not particularly" , "Where soldiers should have faith" .)