Broadcast produced by man of letters in overseas post
I believe the answer is:
air mail
'overseas post' is the definition.
(I've seen this before)
'broadcast produced by man of letters' is the wordplay.
'broadcast' becomes 'air' ('air' can be a synonym of 'broadcast').
'produced by' means one lot of letters go next to another.
'man of letters' becomes 'mail' (I can't justify this - if you can you should believe this answer much more).
'air'+'mail'='AIR-MAIL'
'in' is the link.
Can you help me to learn more?
(Other definitions for air mail that I've seen before include "Post via plane" , "Overseas letter service" , "Overseas postal method" , "Letters carried by plane" , "Flying post?" .)