Region of oxygen gas (5)
I believe the answer is:
ozone
This clue might be a double definition.
'region of oxygen' is the first definition.
'ozone' can be an answer for 'oxygen' (thesaurus). I am unsure of the 'region of' bit.
'gas' is the second definition.
(I've seen this before)
Can you help me to learn more?
(Other definitions for ozone that I've seen before include "Gas forming screen against ultraviolet radiation in the stratosphere" , "Gas formed from oxygen" , "Gas that screens ultraviolet radiation" , "Bracing air of old, now seen as potentially toxic" , "A gas; threatened layer in air" .)