The most common and most likely answer to this crossword clue is the 3 letter word SUE
S
U
E
More clues leading to the result SUE
Buddy explains!

The straight part
We know that "appeal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case appeal = sue.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.
What does SUE mean?
SUE verb- To file a legal action against someone, generally a non-criminal action.
- To seek by request; to make application; to petition; to entreat; to plead.
- (of a hawk) To clean (the beak, etc.).
- To leave high and dry on shore.
- To court.
- To follow.