Form of oxygen (5)
I believe the answer is:
ozone
'form of oxygen' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ozone that I've seen before include "Invigorating air of the seaside" , "Toxic form of oxygen" , "Gas formed from oxygen" , "Seaside air, maybe" , "That precious layer" .)