The most common and most likely answer to this crossword clue is the 15 letter word MERCHANTÂ SEAMEN
I've seen this crossword clue in The Guardian
M
E
R
C
H
A
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T
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S
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M
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Buddy explains!
The straight part
We know that "Long-haul workers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case long-haul workers = merchant seamen.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.