Unsubstantial passage possibly half a page short (5)
I believe the answer is:
gases
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for gases that I've seen before include "Eg, O, H, Ne, CO, etc." , "rabbits" , "Compressible fluids" , "Neon, oxygen, etc" , "Hydrogen and carbon monoxide, say" .)