Wet weather beginning to move on shortly (7)
I believe the answer is:
monsoon
'wet weather' is the definition.
(thesaurus)
'beginning to move on shortly' is the wordplay.
'beginning to' says to take the initial letters.
'shortly' becomes 'soon' (synonyms).
The initial letter of 'move' is 'm'.
'm'+'on'+'soon'='MONSOON'
(Other definitions for monsoon that I've seen before include "Asian wind; rainy season" , "Season of heavy rain" , "Rainy period" , "Seasonal wind (south-west or north-east) of southern Asia" , "Rainy season of India and SE Asia" .)