Defeat of opponent, very, very old country (6)
I believe the answer is:
kosovo
'country' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'defeat of opponent very very old' is the wordplay.
'defeat of opponent' becomes 'ko' (I can't explain this - if you can you should believe this answer much more).
'very' becomes 'so' (I've seen this in other clues).
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ko'+'so'+'v'+'o'='KOSOVO'
Can you help me to learn more?
(Other definitions for kosovo that I've seen before include "Land recently emerging" , "Part of former Yugoslavia" , "Former province of Serbia that declared independence in 2008, capital Pristina" , "Balkans area" , "Balkan state declared independent in 2008" .)