Rainy season from first of May, happening shortly? (7)
I believe the answer is:
monsoon
'rainy season' is the definition.
(monsoon is a kind of rainy season)
'first of may happening shortly?' is the wordplay.
'first of' indicates taking the first letters.
'happening' becomes 'on'.
'shortly?' becomes 'soon' ('soon' can be a synonym of 'shortly').
The first letter of 'may' is 'm'.
'm'+'on'+'soon'='MONSOON'
'from' acts as a link.
(Other definitions for monsoon that I've seen before include "Rain-bringing wind" , "A lot of rain, perhaps" , "Rainy period" , "Asian wind; rainy season" , "Wild weather" .)