The most common and most likely answer to this crossword clue is the 11 letter word CONFEDERATE
I've seen this crossword clue in The Times
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More clues leading to the result CONFEDERATE
Buddy explains!

The straight part
We know that "Allied" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case allied = confederate.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 CONFEDERATE mean?
CONFEDERATE noun- A member of a confederacy.
- An accomplice in a plot.
- An actor who participates in a psychological experiment pretending to be a subject but in actuality working for the researcher (also known as a "stooge").
- To combine in a confederacy.
- Of, relating to, or united in a confederacy
- Banded together; allied.