They're constantly fed by small courses (6)
I believe the answer is:
rivers
'courses' is the definition.
The definition and answer can be both natural objects as well as being plural nouns.
Perhaps they are linked in a way I don't understand?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for rivers that I've seen before include "Thames and Hudson?" , "Comedienne Joan" , "Large natural streams of water" , "''Only Our ... Run Free'' (MacConnell ballad)" , "''Only Our ... Run Free'' (McConnell ballad)" .)