Some nasty weather (5)
I believe the answer is:
sleet
'some nasty weather' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sleet that I've seen before include "Partly melted falling 16 across or 18 down" , "Wintry precipitation" , "Rain/ice mix" , "Freezing rain" , "Precipitate evolution" .)