Note downpour develop over time and where it ends up (8)
I believe the answer is:
drainage
'where it ends up' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'note downpour develop over time' is the wordplay.
'note' becomes 'd' (musical note).
'downpour' becomes 'rain' (I've seen this in another clue).
'develop over time' becomes 'age' (to age is to develop or mature).
'd'+'rain'+'age'='DRAINAGE'
'and' acts as a link.
(Other definitions for drainage that I've seen before include "System of waste channels" , "Water run-off system" , "Evacuation -- gardenia (anag)" , "System to manage waste water" , "Water removal system" .)