
1) European waterway 2) European river 3) European watercourse 4) Polish river 5) Polish waterway 6) Polish watercourse 7) River in Poland 8) River in Europe 9) Waterway in Europe 10) Watercourse in Poland 11) Watercourse in Europe 12) Waterway in Poland 
Found on 
https://www.crosswordclues.com/clue/prosna

 The Prosna is a river in central Poland, a tributary of the Warta river (near Pyzdry), with a length of 217 kilometres (28th longest) and the basin area of 4,925 km2 (all in Poland). ==Towns== ...
Found on 
http://en.wikipedia.org/wiki/Prosna

Prosna is a cultivated variety of potato.
Found on 
http://www.probertencyclopaedia.com/browse/QP.HTM
  No exact match found.