The most common and most likely answer to this crossword clue is the 6 letter word UPCAST
I've seen this crossword clue in The Times
U
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Buddy explains!

The straight part
We know that "mine shaft" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mine shaft = upcast.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 UPCAST mean?
UPCAST noun- A cast; a throw.
- The ventilating shaft of a mine out of which the air passes after having circulated through the mine.
- A current of air passed along such a shaft.
- An upset, as from a carriage.
- A taunt; a reproach.
- A cast from subtype to supertype.
- A message transmitted via upcasting.
- To cast or throw up; to turn upward.
- To taunt; to reproach; to upbraid.
- To cast from subtype to supertype.
- To broadcast a message or data to aircraft or satellites, especially via radio waves; as opposed to uplinking to a specific satellite or aircraft
- Cast up; thrown upward.