The most common and most likely answer to this crossword clue is the 6 letter word CLOSET
I've seen this crossword clue in Financial Times
C
L
O
S
E
T
More clues leading to the result CLOSET
- Lose in court room, small
- Small room near end of apartment
- Secret escape from within court
- Small room near rear of apartment
- Intimate conclusion of plot not revealed
- Near front of tall cupboard
- Clubs don’t keep model secret
- Cupboard shut by last in basement
- Conservative leader to suffer defeat? Time for recess
- Hidden nearby, a slice of toast
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 = closet.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 CLOSET mean?
CLOSET noun- Any private area, (particularly) bowers in the open air.
- Any private or inner room, (particularly):
- A pew or side-chapel reserved for a monarch or other feudal lord.
- A private cabinet, (particularly):
- Any small room or side-room, (particularly):
- An ordinary similar to a bar but half as broad.
- A sewer.
- To shut away for private discussion.
- To put into a private place for a secret interview or interrogation.
- To shut up in, or as in, a closet for concealment or confinement.
- Private.
- Secret, (especially) with reference to gay people who are in the closet; closeted.