German city on the Rhine (4)
'german city on the rhine' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for bonn that I've seen before include "West German capital, 1949-90" , "Former capital of the German Federal Republic" , "Rhine city" , "W German capital 1949-90" , "Beethoven's birthplace, old capital" .)
(Other definitions for dusseldorf that I've seen before include "Capital of North Rhine-Westphalia" , "German city on Rhine" , "Industrial city of North Rhine-Westphalia" .)