There will be wind one day before long (7)
I believe the answer is:
monsoon
'there will be wind' is the definition.
(type of wind in Asia)
'one day before long' is the wordplay.
'one day' becomes 'mon' (abbreviation for Monday).
'before long' becomes 'soon' (synonyms).
'mon'+'soon'='MONSOON'
(Other definitions for monsoon that I've seen before include "Wet seasonal wind" , "Seasonal wind or rainy season in S Asia" , "Rain-bringing weather feature" , "Season of heavy rain" , "A lot of rain, perhaps" .)