The most common and most likely answer to this crossword clue is the 10 letter word MESSENGERS
I've seen this crossword clue in Independent
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Buddy explains!
The straight part
We know that "Couriers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case couriers = messengers.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.
What does MESSENGERS mean?
MESSENGERS noun- One who brings messages.
- A light line with which a heavier line may be hauled e.g. from the deck of a ship to the pier.
- The supporting member of an aerial cable (electric power or telephone or data).
- A person appointed to perform certain ministerial duties under bankrupt and insolvent laws, such as to take charge of the estate of the bankrupt or insolvent.
- An instant messenger program.
- A forerunner.
- A light scudding cloud preceding a storm.
- A piece of paper, etc., blown up a string to a kite.
- A weight dropped down a line to close a Nansen bottle.
- The secretary bird.
- A messenger-at-arms.