It’s to let the rain through when there’s a lot of water (7)
I believe the answer is:
torrent
'a lot of water' is the definition.
I can't judge whether this defines the answer.
'it's to let the rain through when there's' is the wordplay.
'it' becomes 't' (abbreviation. e.g. in 'tis).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'let the rain through' becomes 'torr' (I am not sure about this - if you are sure you should believe this answer much more).
'when' becomes 'e' (I've seen this in other clues).
'there's' becomes 'n' (I've seen this in other clues).
'torr'+'e'+'n'='torren'
't' after 'torren' is 'TORRENT'.
Can you help me to learn more?
(Other definitions for torrent that I've seen before include "Overwhelming outpouring" , "Host" , "Turbulent flow" , "Heavy stream" , "Rush of water" .)