Gas — over one ounce (5)
I believe the answer is:
ozone
'gas' is the definition.
(I know that ozone is a type of gas)
'over one ounce' is the wordplay.
'over' says to put letters next to each other (in a down clue, some letters go over others).
'ounce' becomes 'oz'.
'one' put after 'oz' is 'OZONE'.
(Other definitions for ozone that I've seen before include "Layer that protects against ultraviolet rays" , "Allotrope of oxygen" , "lively atmosphere" , "Gas forming protective layer" , "Gas that screens ultraviolet radiation" .)