Blue gas, a form of oxygen (5)
I believe the answer is:
ozone
I believe this is a double definition.
'blue gas' is the first definition.
(I've seen this in another clue)
'a form of oxygen' is the second definition.
(I've seen this in another clue)
(Other definitions for ozone that I've seen before include "Bracing air (colloq.)" , "(Depleted) layer of gas" , "Colourless gas, O3" , "Form of atmospheric oxygen" , "Condensed oxygen; sea air" .)