Gas region to the south of Ohio (5)
I believe the answer is:
ozone
'gas' is the definition.
(I know that ozone is a type of gas)
'region to the south of ohio' is the wordplay.
'region' becomes 'zone' (zone is a kind of region).
'to the south of' is a charade indicator (letters next to each other).
'ohio' becomes 'o'.
'zone' put after 'o' is 'OZONE'.
(Other definitions for ozone that I've seen before include "Colourless gas soluble in alkalis and cold water" , "It's said to brace" , "Atmospheric layer in some danger" , "Sea air (found in layer?)" , "Gas that screens ultraviolet radiation" .)