The most common and most likely answer to this crossword clue is the 14 letter word JAMESÂ HANRATTY
I've seen this crossword clue in The Guardian
J
A
M
E
S
Â
H
A
N
R
A
T
T
Y
Buddy explains!
The straight part
We know that "Alleged killer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case alleged killer = james hanratty.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.