Capital solo arrangement (4)
I believe the answer is:
oslo
'capital' is the definition.
(capital of Norway)
'solo arrangement' is the wordplay.
'arrangement' indicates anagramming the letters.
'solo' anagrammed gives 'OSLO'.
(Other definitions for oslo that I've seen before include "European capital in go-slow" , "City in Norway" , "Nordic capital" , "Capital of Norway" , "European capital city (found in Czechoslovakia!)" .)