Watercourses new line (4)
I believe the answer is:
nile
'watercourses new line' is the definition.
I can't judge whether this defines the answer.
'new line' is the wordplay.
'new' is an anagram indicator.
'line' anagrammed gives 'NILE'.
Can you help me to learn more?
(Other definitions for nile that I've seen before include "River of Egypt" , "Take a line to the river" , "eastern flower" , "'The longest river, blue and white (4)'" , "River (which is originally blue and white)" .)