Excellent swindle in NI capital (7)
I believe the answer is:
nairobi
'capital' is the definition.
(capital of Kenya)
'excellent swindle in ni' is the wordplay.
'excellent' becomes 'ai' (resembles 'Ar', old-fashioned term for excellent).
'swindle' becomes 'rob' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'ai'+'rob'='airob'
'airob' going inside 'ni' is 'NAIROBI'.
(Other definitions for nairobi that I've seen before include "Flat round object" , "Kenyan city" , "City of Kenya" , "East African capital" , "Badly-behaved child" .)