Nothing's to be seen on the German river (4)
I believe the answer is:
oder
'river' is the definition.
(Oder River is an example)
'nothing's to be seen on the german' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'to be seen on' is a charade indicator (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 "waterway" , "river, one in central Europe" , "East European river" , "German and Polish river" , "River on border of Poland and Germany" .)