Atmospheric gas (5)
'atmospheric gas' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ozone that I've seen before include "Colourless gas, O3" , "Atmospheric layer in some danger" , "Sea-air" , "invigorating British air" , "Shield against radiation" .)
'atmospheric gas' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for oxygen that I've seen before include "A constituent of water" , "publicity, perhaps" , "Gas (of publicity?)" , "Twist together" , "Element essential to combustion" .)