The most common and most likely answer to this crossword clue is the 13 letter word POWER-SHARING
I've seen this crossword clue in The Guardian
P
O
W
E
R
-
S
H
A
R
I
N
G
Buddy explains!
The straight part
We know that "Joining forces" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case joining forces = power-sharing.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 POWER-SHARING mean?
POWER-SHARING noun- The sharing of power by two governments, or two political parties, especially in a disputed territory