The most common and most likely answer to this crossword clue is the 6 letter word PITMAN
I've seen this crossword clue in Independent
P
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M
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Buddy explains!

The straight part
We know that "Underground worker" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case underground worker = pitman.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 PITMAN mean?
PITMAN noun- (plural "pitmen") One who works in a pit, as in mining, in sawing timber, etc.
- (plural "pitmen" or "pitmans") A connecting rod in machinery, especially in a sawmill.