### Clue: Lay armour by post far away (3-4)
### Answer: AIR-MAIL 
 ### Explanation: Definition: by post far away
  AIR(a tune,lay) + MAIL(chain armour of old). 
Answer: Descriptive of say, a letter in the post to be sent to a far away place using air transportation, hence an airmail letter. The references give it without the hyphen, which would have made it harder without the enumeration hint?