Soldier from capital turned on mean country, changing sides (12)
I believe the answer is:
paramilitary
'soldier from capital turned on mean country changing sides' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'soldier from capital turned on mean country changing sides' is the wordplay.
'soldier' becomes 'para' (short for paratrouper).
'from' is a charade indicator (letters next to each other).
'capital turned on mean country changing sides' becomes 'military' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'para'+'military'='PARAMILITARY'
Can you help me to learn more?
(Other definitions for paramilitary that I've seen before include "Member of armed anti-government organisation" , "(Member) of organised armed civilian combat group" .)