So on the day before it's rainy (7)

I believe the answer is:

monsoon

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'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).
'so'+'on'='soon'
'soon' after 'mon' is 'MONSOON'.

Can you help me to learn more?

(Other definitions for monsoon that I've seen before include "Asian wind; rainy season" , "Seasonal wind and rain in S Asia" , "Seasonal Indian Ocean wind" , "Seasonal Asian wind" , "Very heavy rainfall season" .)

I've seen this clue in the Evening Standard.

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