Frozen carbon dioxide (3,3)
I believe the answer is:
dry ice
'frozen 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 "Solidified CO2" , "Theatrical mist" , "Carbon dioxide snow" , "Carbon dioxide vapour used on stage" , "Producer of stage mist" .)