Una's as troublesome in the capital (6)
I believe the answer is:
nassau
'capital' is the definition.
(Nassau is an example)
'una's as troublesome' is the wordplay.
'troublesome' is an anagram indicator.
'unas'+'as'='unasas'
'unasas' with letters rearranged gives 'NASSAU'.
'in the' acts as a link.
(Other definitions for nassau that I've seen before include "Bahamas cap." , "Caribbean resort" , "Bahamian capital" , "Capital of the Bahamas" , "Dublin street in the Bahamas?" .)