The most common and most likely answer to this crossword clue is the 6 letter word YANKEE
I've seen this crossword clue in The Times
Y
A
N
K
E
E
More clues leading to the result YANKEE
- Bet it's American!
- Yikes! No turning back for statesman
- Endless corruption absorbed by the old American
- How shocking vote against withdrawing is for northerner in Civil War
- Zulu comes after this Union soldier
- Bet he's from New England
- Bet there will be no turning back — eager not to stop
- Bet Europeans pursue tug
- Bet made by sports pro in New York
- Bet Uruguayan keeper holds this comfortably in the end
Buddy explains!

The straight part
We know that "Union soldier" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case union soldier = yankee.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 YANKEE mean?
YANKEE noun- A headsail resembling a genoa or a jib but with a high-cut clew, normally used together with a staysail. A sailing boat is typically equipped with three yankee sails of different sizes, number one being the largest.