Old invader captured Hungary's capital (4)
I believe the answer is:
goth
'old invader' is the definition.
(I've seen this before)
'captured hungary's capital' is the wordplay.
'captured' becomes 'got' ('get' can be a synonym of 'capture').
'capital' suggests taking the first letters.
The initial letter of 'hungary' is 'h'.
'got'+'h'='GOTH'
(Other definitions for goth that I've seen before include "Uncivilised type" , "punk style" , "Teutonic invader of the Roman Empire" , "Rock music with gloomy lyrics" , "Youngster clad in black" .)