Millions working early in rainy season (7)
I believe the answer is:
monsoon
'rainy season' is the definition.
(monsoon is a kind of rainy season)
'millions working early' is the wordplay.
'millions' becomes 'm' (abbreviation).
'working' becomes 'on'.
'early' becomes 'soon' (I've seen this before).
'm'+'on'+'soon'='MONSOON'
'in' is the link.
(Other definitions for monsoon that I've seen before include "Seasonal wind or rainy season in S Asia" , "Rain-bringing wind" , "Seasonal wind (south-west or north-east) of southern Asia" , "Wild weather" , "'Seasonal wind in southern Asia, brings heavy rains (7)'" .)