Fog, beginnings of rain and light wind (7)
I believe the answer is:
mistral
'wind' is the definition.
(I know that mistral is a type of boreas)
'fog beginnings of rain and light' is the wordplay.
'fog' becomes 'mist' (mist is a kind of fog).
'beginnings of' indicates taking the first letters.
The first letters of 'rain and light' is 'ral'.
'mist'+'ral'='MISTRAL'
(Other definitions for mistral that I've seen before include "Famous wind in the Rhone Valley blowing towards the Med" , "Violent northeast wind of southern France" , "Strong cold dry wind" , "Strong wind down the 20 valley" , "French wind" .)