The most common and most likely answer to this crossword clue is the 9 letter word SMUGGLERS
I've seen this crossword clue in The Guardian
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More clues leading to the result SMUGGLERS
Buddy explains!
The straight part
We know that "They try to do their running on the quiet /They try to do their running on the quiet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they try to do their running on the quiet /they try to do their running on the quiet = smugglers.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 SMUGGLERS mean?
SMUGGLERS noun- One who smuggles things.
- A vessel employed in smuggling.