The most common and most likely answer to this crossword clue is the 9 letter word MESSENGER
I've seen this crossword clue in The Guardian
M
E
S
S
E
N
G
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R
More clues leading to the result MESSENGER
- Runner turned green after hash
- Dad leaves hitchhiker, say, on Middle East express
- Relatively confused, Italy turns to English news provider
- Courier has difficulty getting green transport
- Envoy to dine with English earl now and then
- Today’s way of keeping in touch with errand-boy?
- Predicament encountered by awfully green go-between …
- Envoy has trouble with green movement
- Litter with green rubbish on Mercury, perhaps
- Mix-up by green cycling courier
Buddy explains!

The straight part
We know that "go-between" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case go-between = messenger.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 MESSENGER mean?
MESSENGER 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.
- To send something by messenger.