A high-flown sort of letter? (3,4)

I believe the answer is:

air mail

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'a high-flown' is the definition.
I can't judge whether this defines the answer.

'sort of letter?' is the wordplay.
'sort of' becomes 'air' (I am not sure about this - if you are sure you should believe this answer much more).
'letter?' 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 "Overseas postal method" , "Letters taken by plane" , "Post via plane" , "Overseas postage" , "Overseas post option" .)

I've seen this clue in the Evening Standard.

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