River in Heidelberg (4)
I believe the answer is:
elbe
'river' is the definition.
(I have seen 'River of Germany ' mean 'elbe' so perhaps 'river' could also mean 'elbe')
'in heidelberg' is the wordplay.
'in' indicates a hidden word.
'ELBE' can be found hidden inside 'heidelberg'.
(Other definitions for elbe that I've seen before include "Flower" , "Current" , "A major German river" , "European river" , "It flows through Germany" .)