The most common and most likely answer to this crossword clue is the 6 letter word LEAGUE
I've seen this crossword clue in Independent
L
E
A
G
U
E
More clues leading to the result LEAGUE
- The French taking over from Victor in leading obscure group
- Amongst les légionnaires the fit ran together for 5km or so
- Maritime measure the French applied to malaria
- Coalition: the French disease
- Group in shelter nursing endless fever
- French article put forward case: right to leave union
- French article on fever in society
- French article unclear, dismissing initial alliance
- Measure of what certain sportsmen hope to win
- Group getting the French fever
Buddy explains!

The straight part
We know that "union" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case union = league.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 LEAGUE mean?
LEAGUE noun- A group or association of cooperating members.
- An organization of sports teams which play against one another for a championship.
- (often in the negative) A class or type of people or things that are evenly matched or on the same level.
- A prefecture-level administrative unit in Inner Mongolia (Chinese: 盟).
- To form an association; to unite in a league or confederacy; to combine for mutual support.
- A version of rugby football, played between two teams of thirteen.
- (measurement) The distance that a person can walk in one hour, commonly taken to be approximately three English miles (about five kilometers).
- A stone erected near a public road to mark the distance of a league.