Two spheres of gas protecting the globe (5)
I believe the answer is:
ozone
'two spheres of' is the definition.
The definition suggests a singular noun which matches the answer.
'gas protecting the globe' is the wordplay.
'gas' becomes 'O' (chemical symbol for oxygen).
'protecting the globe' becomes 'zone' (I can't justify this - if you can you should believe this answer much more).
'o'+'zone'='OZONE'
Can you help me to learn more?
(Other definitions for ozone that I've seen before include "Gas formed from oxygen" , "Form of oxygen in earth's upper atmosphere - there's a hole in it" , "Condensed oxygen; sea air" , "something found in the atmosphere" , "It's said to brace" .)