Excellent plunder in some British region's capital (7)
I believe the answer is:
nairobi
'capital' is the definition.
(Nairobi is an example)
'excellent plunder in some british region's' is the wordplay.
'excellent' becomes 'ai'.
'plunder' becomes 'rob' (I've seen this before).
'in' is an insertion indicator.
'some british region's' becomes 'ni' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ai'+'rob'='airob'
'airob' put inside 'ni' is 'NAIROBI'.
Can you help me to learn more?
(Other definitions for nairobi that I've seen before include "Kenya's capital city" , "Foreign capital" , "Kenyan capital" , "African city" , "I, a robin (anag) - city" .)