River in countryside with source hidden (4)
I believe the answer is:
ural
'river' is the definition.
(I've seen this before)
'countryside with source hidden' is the wordplay.
'countryside' becomes 'rural' (I've seen this before).
'with source hidden' means to remove the first letter.
'rural' with its first letter taken away is 'URAL'.
'in' is the link.
(Other definitions for ural that I've seen before include "Asian flower" , "Russian river giving name to mountains" , "Europe/Asia river" , "Russian river flowing into the Caspian Sea" , "Mountain range between Europe and Asia" .)