capital city on the Danube (6)
I believe the answer is:
vienna
'capital city on the danube' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for vienna that I've seen before include "Capital on the Danube Canal" , "sort of sausage" , "On the Danube, it was home to many great composers" , "Austrian city of the waltz" , "Capital city on the Danube" .)