The most common and most likely answer to this crossword clue is the 7 plus 8 letter word DISTANT RELATION
I've seen this crossword clue in The Guardian
D
I
S
T
A
N
T
R
E
L
A
T
I
O
N
Buddy explains!
The straight part
We know that "one not in my own branch?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one not in my own branch? = distant relation.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.