The most common and most likely answer to this crossword clue is the 3 letter word PUP
I've seen this crossword clue in The Times
P
U
P
More clues leading to the result PUP
Buddy explains!

The straight part
We know that "Young setter, maybe" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case young setter, maybe = pup.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 PUP mean?
PUP noun- A young dog, wolf, fox, seal, or shark, or the young of certain other animals.
- A young, inexperienced person.
- Any cute dog, regardless of age.
- A short semi-trailer used jointly with a dolly and another semi-trailer to create a twin trailer.
- To give birth to pups.