Nothing to be seen on the German river (4)
I believe the answer is:
oder
'river' is the definition.
(River Oder in central Europe)
'nothing to be seen on the german' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'to be seen on' says to put letters next to each other.
'the german' becomes 'der' ('the' in German).
'o'+'der'='ODER'
(Other definitions for oder that I've seen before include "Polish/German river" , "German and Polish river" , "river, one in central Europe" , "flowing water" , "German banker" .)