Successor man announced in letter? (7)

I believe the answer is:

airmail

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'letter?' is the definition.
(airmail letter is a kind of letter)

'successor man announced' is the wordplay.
'successor' becomes 'air' (I can't explain this - if you can you should believe this answer much more).
'man' becomes 'male'.
'announced' shows a homophone (sound like).
'male' sounds like 'mail'.
'air'+'mail'='AIRMAIL'

'in' is the link.

Can you help me to learn more?

(Other definitions for airmail that I've seen before include "Postal service" , "Letter delivery system" , "Letters sent up" , "It's marked "par avion"" , "Letters that are flown in" .)

I've seen this clue in The Times.

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