It will bring the rain one day shortly (7)
I believe the answer is:
monsoon
'it will bring the rain one' is the definition.
The definition suggests a singular noun which matches the answer.
'day shortly' is the wordplay.
'day' becomes 'mon' (abbreviation for Monday).
'shortly' becomes 'soon' (synonyms).
'mon'+'soon'='MONSOON'
Can you help me to learn more?
(Other definitions for monsoon that I've seen before include "Seasonal wind or rainy season in S Asia" , "Seasonal Indian Ocean wind" , "'Seasonal wind in southern Asia, brings heavy rains (7)'" , "A lot of rain, perhaps" , "Rainy period" .)