The most common and most likely answer to this crossword clue is the 2 plus 3 plus 3 plus 5 letter word BE OUT FOR BLOOD
B
E
O
U
T
F
O
R
B
L
O
O
D
Buddy explains!
The straight part
We know that "to seek vengeance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to seek vengeance = be out for blood.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.