The most common and most likely answer to this crossword clue is the 6 plus 8 letter word DECREE ABSOLUTE
I've seen this crossword clue in The Guardian
D
E
C
R
E
E
A
B
S
O
L
U
T
E
More clues leading to the result DECREE ABSOLUTE
Buddy explains!
The straight part
We know that "end of alliance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case end of alliance = decree absolute.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.