Triatomic form of oxygen (5)
I believe the answer is:
ozone
'triatomic form of oxygen' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ozone that I've seen before include "Layer that protects against ultraviolet rays" , "By sea, you could take in this" , "lively atmosphere" , "Sea air (found in layer?)" , "Bracing air (colloq.)" .)