Over one ounce of oxygen (5)
I believe the answer is:
ozone
'oxygen' is the definition.
(thesaurus)
'over one ounce' is the wordplay.
'over' means one lot of letters go next to another.
'ounce' becomes 'oz'.
'one' put after 'oz' is 'OZONE'.
'of' is the link.
(Other definitions for ozone that I've seen before include "Trioxygen" , "Colourless gas soluble in alkalis and cold water" , "something refreshing" , "Protection" , "Condensed oxygen; sea air" .)