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Road train covering miles for speedy delivery (7)

Ross

I believe the answer is:

airmail

I'm a little stuck... Click here to teach me more about this clue!

'speedy delivery' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe they are linked in a way I don't understand?

'road train covering miles' is the wordplay.
'road' becomes 'AI' (resembles A1, a major UK road).
'train' becomes 'rail' (I've seen this in another clue).
'covering' means one lot of letters goes inside another.
'miles' becomes 'm' (common abbreviation).
'rail' enclosing 'm' is 'rmail'.
'ai'+'rmail'='AIRMAIL'

'for' is the link.

Can you help me to learn more?

(Other definitions for airmail that I've seen before include "Quick way to deliver" , "Flying post?" , "Speedy deliveries" , "Flying service" , "Letters carried by plane" .)

I've seen this clue in The Telegraph.
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