The most common and most likely answer to this crossword clue is the 12 letter word COLLABORATED
I've seen this crossword clue in The Guardian
C
O
L
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A
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Buddy explains!
The straight part
We know that "Joined forces with" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case joined forces with = collaborated.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 COLLABORATED mean?
COLLABORATED verb- To work together with others to achieve a common goal.
- To voluntarily cooperate treasonably, as with an enemy occupation force in one's country.