The most common and most likely answer to this crossword clue is the 6 letter word TEAMED
I've seen this crossword clue in The Daily Telegraph
T
E
A
M
E
D
Buddy explains!

The straight part
We know that "United" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case united = teamed.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 TEAMED mean?
TEAMED verb- To form a group, as for sports or work.
- (by extension) To go together well; to harmonize.
- To convey or haul with a team.
- To form together into a team.
- To give work to a gang under a subcontractor.
- Associated with others in a team