### Clue: Air retains gases, primarily Oxygen, Nitrogen and ___? (5)
### Answer: ARGON 
 ### Explanation: Definition: ___
 **The 1st letters of, respectively,**(…, primarily) "Air retains gases " + O(the chemical symbol for the element, oxygen) + N(likewise for nitrogen).
Defn: Fill in the blank with a naturally occurring component of the air around us, together with oxygen, nitrogen and some other gases.