The most common and most likely answer to this crossword clue is the 4 letter word SIRS
I've seen this crossword clue in The Daily Telegraph
S
I
R
S
Buddy explains!

The straight part
We know that "Gentlemen" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gentlemen = sirs.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 SIRS mean?
SIRS noun- A man of a higher rank or position.
- A respectful term of address to a man of higher rank or position, particularly:
- A respectful term of address to an adult male (often older), especially if his name or proper title is unknown.
- To address (someone) using "sir".