Atmosphere layer over region (5)
I believe the answer is:
ozone
'atmosphere layer' is the definition.
(I've seen this in another clue)
'over region' is the wordplay.
'over' becomes 'o' (cricket abbreviation).
'region' becomes 'zone' (zone is a kind of region).
'o'+'zone'='OZONE'
(Other definitions for ozone that I've seen before include "lively atmosphere" , "Gas formed from oxygen" , "A gas; threatened layer in air" , "Form of oxygen, O3" , "Condensed oxygen; sea air" .)