Ground about to be swamped by boundless watercourse (5)
I believe the answer is:
arena
'ground' is the definition.
(I've seen this before)
'to be swamped by boundless watercourse' is the wordplay.
'to be' becomes 'are' (I've seen this in another clue).
'swamped by boundless watercourse' becomes 'na' (I am not sure about this - if you are sure you should believe this answer much more).
'are'+'na'='ARENA'
'about' acts as a link.
Can you help me to learn more?
(Other definitions for arena that I've seen before include "Area of activity, boxing perhaps" , "Ring; stage" , "Amphitheatre's central space" , "Area of combat" , "Wembley, for example" .)