City on the Rhine (5)
'city on the rhine' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for basel that I've seen before include "In European city" , "City of Switzerland" , "Swiss city" .)
(Another definition for mainz that I've seen is " port in Germany".)
(Other definitions for arnhem that I've seen before include "Location of battle" , "Second World War battle" , "Dutch city" , "Netherlands scene of 1944 battle" , "place noted for fighting" .)