High-flown letters (3,4)
I believe the answer is:
air mail
'high-flown letters' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'flown letters' is the wordplay.
'flown' becomes 'air' (I can't justify this - if you can you should give a lot more credence to this answer).
'letters' becomes 'mail' (I've seen this before).
'air'+'mail'='AIR-MAIL'
Can you help me to learn more?
(Other definitions for air mail that I've seen before include "Letters that are flown in" , "Postal service" , "Overseas postal method" , "Overseas postage" , "Letters taken by plane" .)