The most common and most likely answer to this crossword clue is the 10 letter word PINSTRIPES
I've seen this crossword clue in The Guardian
P
I
N
S
T
R
I
P
E
S
Buddy explains!
The straight part
We know that "Old habit of city gents" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old habit of city gents = pinstripes.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 PINSTRIPES mean?
PINSTRIPES noun- A very thin stripe on a fabric.
- Such a fabric.
- A suit made of such fabric.