Pest you’ll find on this flower (6)
I believe the answer is:
danube
I believe this is a double definition.
'pest you'll find on this' is the first definition.
(Budapest is on the Danube)
'flower' is the second definition.
(European river - something which flows)
(Other definitions for danube that I've seen before include "It's blue according to the song" , "Viennese river" , "River flowing through Belgrade, Budapest and Vienna" , "River in Europe" , "South European river" .)