Solid carbon dioxide (3,3)
I believe the answer is:
dry ice
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">solid carbon dioxide</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)