The most common and most likely answer to this crossword clue is the 16 letter word CLOAK-AND-DAGGER
I've seen this crossword clue in Independent
C
L
O
A
K
-
A
N
D
-
D
A
G
G
E
R
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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 CLOAK-AND-DAGGER mean?
CLOAK-AND-DAGGER adjective- Marked by menacing furtive secrecy, often with a melodramatic tint or espionage involved.