Solid carbon dioxide (3,3)
I believe the answer is:
dry ice
'solid carbon dioxide' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dry ice that I've seen before include "Carbon dioxide snow" , "Substance used for misty stage effects" , "Theatrical mist" , "may be used for effect on stage" , "White mist on stage" .)