Colourless gas, O3 (5)
I believe the answer is:
ozone
This clue is a double definition.
'colourless gas' is the first definition.
(I've seen this before)
'o3' is the second definition.
(I've seen this in another clue)
(Other definitions for ozone that I've seen before include "Sea air; toxic gas" , "Sea air (found in layer?)" , "Toxic form of oxygen" , "Atmospheric layer in some danger" , "Gas; bracing sea air" .)