The most common and most likely answer to this crossword clue is the 15 letter word UNDER-THE-TABLE
U
N
D
E
R
-
T
H
E
-
T
A
B
L
E
Buddy explains!
The straight part
We know that "in a clandestine manner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in a clandestine manner = under-the-table.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 UNDER-THE-TABLE mean?
UNDER-THE-TABLE adjective- Alternative spelling of under the table; in attributive use.