First of mud appearing before long in rainy season (7)
I believe the answer is:
monsoon
'rainy season' is the definition.
(monsoon is a kind of rainy season)
'first of mud appearing before long' is the wordplay.
'first of' suggests taking the first letters.
'appearing' becomes 'on' (e.g. on stage or on television).
'before long' becomes 'soon' (I've seen this before).
The initial letter of 'mud' is 'm'.
'm'+'on'+'soon'='MONSOON'
'in' acts as a link.
(Other definitions for monsoon that I've seen before include "Asian wind; rainy season" , "Rain-bringing wind" , "Rainy season of India and SE Asia" , "Seasonal wind or rainy season in S Asia" , "A lot of rain, perhaps" .)