The most common and most likely answer to this crossword clue is the 4 letter word CADS
C
A
D
S
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The straight part
We know that "No gentlemen" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case no gentlemen = cads.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 CADS mean?
CADS noun- A low-bred, presuming person; a mean, vulgar fellow.
- A person who stands at the door of an omnibus to open and shut it, and to receive fares; an idle hanger-on about innyards.