Content of tube fills European river (6)
I believe the answer is:
danube
'river' is the definition.
(I know that Danube is a river)
'content of tube fills european' is the wordplay.
'content of' says to take the centre.
'fills' indicates putting letters inside.
'european' becomes 'Dane' (someone from Denmark).
The central letters of 'tube' are 'ub'.
'ub' inserted within 'dane' is 'DANUBE'.
(Other definitions for danube that I've seen before include "Ban due (anag) - river" , "Central European river" , "River flowing through Belgrade, Budapest and Vienna" , "Europe's musically blue river" , "River flowing from Black Forest to Black Sea" .)