Hints: Letters from the Expedition

Getting started:
These are cryptic crossword clues without the definition part. You should try to solve them, even if the answers don’t look like common words at first. All of the answers do have something in common, which is hinted at by the title.
Example clue:
In the middle of something = ETH
Stuck on extraction:
The answers are all letters, although perhaps not the type you will be most familiar with.