Tune man heard is in the post (7)
I believe the answer is:
airmail
'the post' is the definition.
(airmail is a kind of post)
'tune man heard' is the wordplay.
'tune' becomes 'air' (air can mean a piece of music).
'man' becomes 'male'.
'heard' shows a homophone (sound like).
'male' sounds like 'mail'.
'air'+'mail'='AIRMAIL'
'is in' is the link.
(Other definitions for airmail that I've seen before include "Letters taken by plane" , "overseas post?" , "It's marked "par avion"" , "Flying service" , "post far away" .)