The most common and most likely answer to this crossword clue is the 12 letter word UNILATERALLY
I've seen this crossword clue in The Sunday Telegraph
U
N
I
L
A
T
E
R
A
L
L
Y
More clues leading to the result UNILATERALLY
Buddy explains!

The straight part
We know that "not acting with others" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not acting with others = unilaterally.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 UNILATERALLY mean?
UNILATERALLY adverb- In a unilateral way.