Complex 'neath southern capital city (6)
I believe the answer is:
athens
'capital city' is the definition.
(I've seen this before)
'complex 'neath southern' is the wordplay.
'complex' indicates an anagram.
'southern' becomes 's' (abbreviation**).
'neath' with letters rearranged gives 'athen'.
'athen'+'s'='ATHENS'
(Other definitions for athens that I've seen before include "Greek capital" , "EU capital" , "Hasten (anag) -- capital city" , "City of the Parthenon" , "Hasten to Greece" .)