Solid carbon dioxide (3,3)
I believe the answer is:
dry ice
'solid carbon dioxide' is the definition.
(I've seen this in another clue)
This is the entire 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" .)