Copenhagen citizen maybe eating grub half ignored famous river (6)
I believe the answer is:
danube
'famous river' is the definition.
(European river)
'copenhagen citizen maybe eating grub half ignored' is the wordplay.
'copenhagen citizen maybe' becomes 'Dane' (Copenhagen is in Denmark).
'eating' is an insertion indicator.
'half ignored' indicates taking half.
'grub' with half the letters taken is 'ub'.
'dane' placed around 'ub' is 'DANUBE'.
(Other definitions for danube that I've seen before include "South European river" , "River in Europe" , "flower" , "Great European river, musically blue (6)" , "Europe's musically blue river" .)