### Clue: One's put on weight – it's something in the air (5)
### Answer: OZONE 
 ### Explanation: Definition: it's something in the air
  OZ(abbrev. for ounce,unit of weight in the Imperial system) **placed before**(put on) ONE
Defn:  An allotrope of the element oxygen with 3 instead of the common 2 atoms in a molecule, great for protecting earthly life against cosmic rays.