Gaseous substance (6)
I believe the answer is:
vapour
'gaseous substance' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for vapour that I've seen before include "Particles suspended in air" , "Visible steam or clouds" , "Gas-like mist" , "Air-like substance created by heating, like gas" , "Boast" .)