Half of Budapest spread around north European river (6)
I believe the answer is:
danube
'river' is the definition.
(Danube River is an example)
'half of budapest spread around north european' is the wordplay.
'half of' means to take half the letters.
'spread' is an anagram indicator.
'around' means one lot of letters goes inside another.
'north' becomes 'n' (abbreviation).
'european' becomes 'E' (abbreviation e.g. EU).
'budapest' halved is 'buda'.
'buda' with letters rearranged gives 'daub'.
'daub' enclosing 'n' is 'danub'.
'danub'+'e'='DANUBE'
(Other definitions for danube that I've seen before include "Central European river" , "It's blue according to the song" , "Vienna's river" , "Ban due (anag) - river" , "River flowing from Black Forest to Black Sea" .)