Note river without end (8)
I believe the answer is:
terminus
'end' is the definition.
(terminus is a kind of end)
'note river without' is the wordplay.
'note river' becomes 'ter' (I can't explain this - if you can you should give a lot more credence to this answer).
'without' becomes 'minus' (I've seen this before).
'ter'+'minus'='TERMINUS'
Can you help me to learn more?
(Other definitions for terminus that I've seen before include "Final stop" , "Limit" , "final point" , "Victoria, perhaps" , "Final station on bus route" .)