Line in earth where river has disappeared (4)
I believe the answer is:
oche
'line' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
(Other definitions for oche that I've seen before include "Line from which a dart player throws" , "Dart player's mark" , "Dart-playing line" , "Behind which a darts player must stand" , "Line from which dart-player throws" .)