Large natural watercourse (5)
I believe the answer is:
river
(Other definitions for river that I've seen before include "Natural flow of water" , "Eg, Thames or Nile" , "x or y, so to speak" , "course of 1d [ADVENTURE]" , "Type of watercourse" .)
I believe the answer is:
river
(Other definitions for river that I've seen before include "Natural flow of water" , "Eg, Thames or Nile" , "x or y, so to speak" , "course of 1d [ADVENTURE]" , "Type of watercourse" .)