The most common and most likely answer to this crossword clue is the 6 plus 6 letter word SETTLE SCORES
S
E
T
T
L
E
S
C
O
R
E
S
More clues leading to the result SETTLE SCORES
Buddy explains!
The straight part
We know that "Gain revenge" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gain revenge = settle scores.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.