The most common and most likely answer to this crossword clue is the 12 letter word DOOR-TO-DOOR
I've seen this crossword clue in The Guardian
D
O
O
R
-
T
O
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D
O
O
R
Buddy explains!
The straight part
We know that "Type of salesman/non-stop transport service" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case type of salesman/non-stop transport service = door-to-door.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
Crossword Answer definitions
DOOR-TO-DOOR adjective- Going from house to house, when selling, delivering or asking for something.
- Sent or delivered from a store or factory directly to a house.