Carbon copy covering unknown meteorological event (7)
I believe the answer is:
cyclone
'meteorological event' is the definition.
'carbon copy covering unknown' is the wordplay.
'carbon' becomes 'c' ('C' can be a synonym of 'carbon').
'copy' becomes 'clone' (cloning is a kind of copying**).
'covering' is an insertion indicator.
'unknown' becomes 'y' (algebra).
'c'+'clone'='cclone'
'cclone' going around 'y' is 'CYCLONE'.
(Other definitions for cyclone that I've seen before include "lots of wind" , "Violent windstorm" , "Wind spiral" , "Low pressure area or storm" , "Tornado" .)