Mostly popular army officer (2,7)
I believe the answer is:
in general
'mostly' is the definition.
(I've seen this before)
'popular army officer' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'army officer' becomes 'general' (type of army officer).
'in'+'general'='IN GENERAL'
(Other definitions for in general that I've seen before include "More-or-less" , "by and large" , "Mostly" , "Usually; mainly" , "Largely" .)