It's part of the atmosphere when Australia takes on England's top side (5)
I believe the answer is:
ozone
'it's part of the atmosphere' is the definition.
(as in the ozone layer)
'australia takes on england's top side' is the wordplay.
'australia' becomes 'oz' (informal name for Australia).
'takes' says to put letters next to each other.
'on' is a charade indicator (letters next to each other).
'england' becomes 'E' (short for England).
'top side' becomes 'on' (I've seen this in other clues. I am not sure about the 'top' bit.).
'e' put after 'on' is 'one'.
'oz' next to 'one' is 'OZONE'.
'when' is the link.
Can you help me to learn more?
(Other definitions for ozone that I've seen before include "Clean, bracing air" , "bracing atmosphere" , "Sea air (found in layer?)" , "Condensed oxygen; sea air" , "Toxic form of oxygen" .)