Crossword Genius

Correspondence on the up? (7)

Ross

I believe the answer is:

airmail

'correspondence' is the definition.
(thesaurus)

I can't explain the rest of the clue.

(Other definitions for airmail that I've seen before include "It's marked "par avion"" , "Quick way to deliver" , "Letters sent up" , "Speedy deliveries" , "Postal service" .)

I've seen this clue in The Times.
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