The most common and most likely answer to this crossword clue is the 3 plus 2 plus 5 letter word ACE OF CLUBS
I've seen this crossword clue in Financial Times
A
C
E
O
F
C
L
U
B
S
More clues leading to the result ACE OF CLUBS
Buddy explains!
The straight part
We know that "One in black suit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one in black suit = ace of clubs.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.