The most common and most likely answer to this crossword clue is the 3 plus 5 plus 4 plus 2 plus 7 plus 5 plus 6 letter word ONE MAN’S MEAT IS ANOTHER MAN’S POISON
I've seen this crossword clue in Financial Times
O
N
E
M
A
N
â
€
™
S
M
E
A
T
I
S
A
N
O
T
H
E
R
M
A
N
â
€
™
S
P
O
I
S
O
N
Buddy explains!
The straight part
We know that "Why his beef is not with me" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case why his beef is not with me = one man’s meat is another man’s poison.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.