The most common and most likely answer to this crossword clue is the 13 letter word CO-RESPONDENT
I've seen this crossword clue in The Times
C
O
-
R
E
S
P
O
N
D
E
N
T
More clues leading to the result CO-RESPONDENT
Buddy explains!
The straight part
We know that "person in suit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case person in suit = co-respondent.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.