Beginning to move on shortly it's the rainy season (7)
I believe the answer is:
monsoon
'the rainy season' is the definition.
(monsoon is a kind of rainy season)
'beginning to move on shortly' is the wordplay.
'beginning to' says to take the initial letters.
'shortly' becomes 'soon' ('soon' can be a synonym of 'shortly').
The initial letter of 'move' is 'm'.
'm'+'on'+'soon'='MONSOON'
'it's' acts as a link.
(Other definitions for monsoon that I've seen before include "Asian wind; rainy season" , "Seasonal wind or rainy season in S Asia" , "Seasonal wind and rain in S Asia" , "Seasonal wind (south-west or north-east) of southern Asia" , "Very heavy rainfall season" .)