The most common and most likely answer to this crossword clue is the 10 letter word COALESCENT
C
O
A
L
E
S
C
E
N
T
Buddy explains!

The straight part
We know that "Making united" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case making united = coalescent.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 COALESCENT mean?
COALESCENT noun- Any agent that causes coalescence.
- Causing coalescence.