So on the day before it's rainy (7)
I believe the answer is:
'it's rainy' is the definition.
I can't judge whether this definition defines the answer.
'so on the day before' is the wordplay.
'the day' becomes 'mon' (abbreviation for Monday).
'before' is a charade indicator (letters next to each other).
'soon' after 'mon' is 'MONSOON'.
Can you help me to learn more?
(Other definitions for monsoon that I've seen before include "Season of heavy rain", "Asian wind; rainy season", "A lot of rain, perhaps", "Wet seasonal wind", "Seasonal wind (south-west or north-east) of southern Asia".)