The most common and most likely answer to this crossword clue is the 6 letter word COVERT
I've seen this crossword clue in Financial Times
C
O
V
E
R
T
More clues leading to the result COVERT
- Secret deliveries taken into court
- Concealed hide close to nest
- Concealed chap by banks of rivulet
- Knight leaving to switch allegiance in secret
- Insurance agent’s final secret
- Charlie’s open secret
- Secret past revealed in court
- Overlapping screen open, revealing secret
- Secretive clergyman returns joint to army chief
- Concealed panic finally evident
Buddy explains!

The straight part
We know that "secret" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case secret = covert.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 COVERT mean?
COVERT noun- A covering.
- A disguise.
- A hiding place.
- Area of thick undergrowth where animals hide.
- A feather that covers the bases of flight feathers.
- Hidden, covered over; overgrown, sheltered.
- Secret, surreptitious, concealed.