What fills this after downpour's onset? (5)
I believe the answer is:
drain
'onset?' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'what fills this after downpour's' is the wordplay.
I cannot quite understand how this works, but
'downpour' could be 'rain' (pouring rain) and 'rain' is located in the answer.
A single letter 'd' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for drain that I've seen before include "Waste channel/pipe" , "Deprive gradually of strength or resources" , "Nadir (anag.)" , "Empty of liquid" , "See 11 dn" .)